188 Fun Facts About Ocean

About what share of Earth's surface is dry land?

About 29% of Earth's surface is dry land; the ocean covers the other 71%, and because it is deep as well as wide, it is by far the largest living space on the planet.

Which traditional unit of six feet do sailors use to measure the depth of water?

Fathom comes from an Old English word for outstretched arms, because one fathom is roughly the span between a grown man's fingertips.

Roughly how deep is the ocean on average?

About 3.7 kilometres: sunlight fades out within the first 1,000 metres, so most of the seabed lies in permanent darkness.

What is the average salinity of ocean water?

About 3.5%: every kilogram of seawater holds some 35 grams of dissolved salts, most of them worn out of rocks on land by rain and carried down to the sea by rivers.

Which body of water experiences the highest tidal range on Earth?

The Bay of Fundy, between Nova Scotia and New Brunswick, has a tidal range of about 16 metres; its funnel shape makes the basin resonate with the tide.

Divers who breathe compressed air and surface too fast get 'the bends' when which gas bubbles out of their blood?

Nitrogen dissolves into the blood and tissues under pressure and comes out as bubbles if a diver rises too quickly, causing joint pain that can be severe.

What do we call the extra-high tides that arrive when the Sun and Moon line up with Earth?

Spring tides have nothing to do with the season: they come about every two weeks all year round, at new and full moon, when the pulls of the Moon and Sun line up.

What is the global ocean 'conveyor belt' of density-driven deep currents called?

Thermohaline circulation is named for heat and salt: cold, salty water is dense, so it sinks near the poles and creeps along the seabed, drawing warmer surface water in behind it.

Which warm ocean current flows northward along the eastern coast of the United States?

Gulf Stream: Benjamin Franklin, puzzled that mail ships from England were taking weeks longer than they should, had his whaling-captain cousin Timothy Folger chart the current so captains could avoid it.

Males of which whale species are known for singing long, complex songs?

Humpback whale songs were released as an album in 1970; Songs of the Humpback Whale became the best-selling nature recording ever and helped launch the Save the Whales movement.

Most of the animals filmed in the open water off Monterey, California, share which ability?

Bioluminescence: about three-quarters of them can make their own light, and many use it to hide, glowing along the belly to match the light from above so that hunters below see no silhouette.

What is the primary compound that makes seawater salty?

Sodium chloride is ordinary table salt; its two ions dominate seawater because they stay dissolved far longer than most other elements, which settle out or are used up much sooner.

Which ocean zone is known as the "twilight zone" because only faint light reaches it?

The mesopelagic zone runs from about 200 to 1,000 metres down; every night many of its animals swim up to feed near the surface and sink back before dawn, the greatest migration on Earth.

Which mineral compound, also the stuff of seashells, do corals secrete to build the Great Barrier Reef?

Calcium carbonate: each tiny coral animal lays down a hard limestone cup beneath itself, and over thousands of years billions of them have built the largest structure made by living things.

Which penguin, the largest living species, breeds on the Antarctic ice in the depths of winter?

Emperor penguin males spend the dark Antarctic winter balancing a single egg on their feet for about nine weeks without eating, living off their fat until the chick hatches.

By mid-2025, about how much of the ocean floor had been mapped in detail by sonar?

About 25%: the Seabed 2030 project counted 27.3% mapped by June 2025. Satellites have charted the rest only in blurry outline, so whole seamounts and shipwrecks still lie unseen.

Which gas does the ocean absorb from the atmosphere, contributing to ocean acidification?

Carbon dioxide reacts with seawater to form carbonic acid, and the more acidic water makes it harder for oysters, clams and some plankton to build and keep their shells.

Which explorer made the first solo descent to the bottom of the Mariana Trench in 2012?

James Cameron, better known for directing Titanic and Avatar, piloted Deepsea Challenger alone to the Challenger Deep in March 2012 and spent several hours exploring the bottom.

Which is the largest animal known to have ever lived on Earth?

Blue whale: the giant feeds almost entirely on krill, shrimp-like animals only a few centimetres long.

What do female deep-sea anglerfish use their glowing head bulb for?

Attracting prey: the glow comes from light-making bacteria that live inside the bulb, called the esca, and animals drawn to it in the dark are snapped up.

Which of the world's oceanic divisions is the smallest by area?

The Arctic Ocean is also the shallowest of the five and the least salty, because great rivers pour fresh water into it and little evaporates in the cold.

The Mid-Atlantic Ridge rises above the sea to form which island country?

Iceland straddles the ridge, which spreads by about 2.5 centimetres a year on average, so the island itself is slowly splitting between the North American and Eurasian plates.

What is a tsunami primarily caused by?

Most tsunamis are triggered by underwater earthquakes that suddenly shift the seabed; the word is Japanese for 'harbour wave', and in open ocean the wave can pass under a ship almost unnoticed.

What is the term for the partly enclosed coastal water where a river meets the sea and fresh water mixes with salt?

Estuary: the mix of fresh and salt water makes estuaries among the most productive habitats on Earth, nurseries for fish and feeding stops for migrating birds.

What is the Ring of Fire?

A belt of earthquakes and volcanoes rimming the Pacific, the Ring of Fire traces where ocean plates dive under their neighbours; about 90% of the world's earthquakes strike along it.

What are hydrothermal vents?

Seabed fissures that discharge geothermally heated water, sometimes above 350°C, and the bacteria there live on the chemicals in it rather than sunlight. Those bacteria feed the tube worms, clams and shrimp.

What type of organisms form the base of most ocean food chains?

Phytoplankton are microscopic photosynthetic organisms that form the foundation of nearly all marine food webs. They produce roughly 50% of the world's oxygen through photosynthesis, making them critical for life on Earth.

What is the Great Pacific Garbage Patch?

A gyre of drifting debris trapped by currents, not an island: most of its pieces are tiny and hang at or just below the surface, so it does not show up in satellite photos.

What is the Sargasso Sea unique for among the world's seas?

It has no land boundaries: four ocean currents alone mark its edges, and hatchling sea turtles use its floating mats of Sargassum weed as a nursery.

Which shipboard instrument replaced the weighted lead line for measuring water depth?

The echo sounder times a sound pulse's round trip to the seabed, giving a depth in seconds, where lowering a weighted line to the bottom and hauling it back was slow work.

Which wind-driven process feeds the world's richest anchovy and sardine fisheries?

Upwelling happens when winds push surface water away from a coast and cold, nutrient-rich water rises to replace it, fertilising the plankton that feeds huge shoals of fish.

The deepest ocean zone, found only in trenches below 6,000 metres, is named after which Greek god?

Hades, god of the underworld, gives the hadal zone its name; the term was coined in the 1950s, when expeditions found that animals in the trenches differ from those on the abyssal plains above.

Which two people made the first descent to the bottom of the Mariana Trench in 1960?

Jacques Piccard and Don Walsh reached the bottom on 23 January 1960 but stayed only 20 minutes, after a window had cracked on the way down.

What is the name of the symbiotic algae that live inside coral tissue and give coral its color?

Zooxanthellae feed their coral hosts with up to 90% of the energy they need; when overheated corals expel the algae, the reef turns ghostly white, which is coral bleaching.

Point Nemo, the ocean point farthest from any land, has picked up which grim nickname?

Point Nemo is the spacecraft cemetery: hundreds of dead satellites and the Mir space station were steered to re-enter over it, and the nearest humans are often astronauts passing overhead.

What is a submarine canyon?

A submarine canyon is a steep-sided valley cut into the continental slope; many were carved not by rivers but by turbidity currents, swift avalanches of muddy water running downhill.

Which climate pattern warms the surface of the tropical Pacific every few years and shifts weather worldwide?

El Niño means 'the boy' in Spanish: Peruvian fishermen named the warm water after the Christ child because it tended to arrive around Christmas.

The brown pigment sepia was originally made from the ink of which sea creature?

Cuttlefish ink gave the colour its name: sepia is the Greek and Latin word for the animal, and its rich brown ink was used for writing and drawing long before the word passed to old photographs.

Which extinct marine predator had teeth up to 18 centimeters long?

Megalodon, a giant shark that lived from about 23 to 3.6 million years ago, is known almost entirely from its teeth, so its true size remains uncertain.

What is the continental shelf?

The continental shelf is the gently sloping submerged edge of a continent; during the ice ages, when sea level was far lower, much of it was dry land.

Which creature is known as the largest invertebrate on Earth?

Colossal squid: the heaviest ever landed, a 495 kg female caught in Antarctic waters, is kept at New Zealand's Te Papa museum, and larger beaks found in whale stomachs show the species grows bigger still.

Which effect of Earth's rotation bends ocean currents to the right in the Northern Hemisphere?

Coriolis effect: it turns moving water right in the north and left in the south, yet it is far too weak to decide which way a sink or bathtub drains, a popular myth.

Robert Ballard found the Titanic in 1985 after a secret US Navy mission to inspect what?

Nuclear submarines: the Navy sent Ballard's towed camera sled to survey the wrecks of USS Thresher and USS Scorpion, and he found the liner in the 12 days left over.

Which of these seas has the saltiest water?

The Red Sea's salinity reaches about 40 parts per thousand, well above the open ocean's, because it is hot, nearly enclosed and fed by no major rivers.

Which disease of long sea voyages did the Royal Navy fight by adding citrus juice to sailors' rations?

Scurvy comes from a lack of vitamin C; the lime juice issued on British warships to prevent it earned British sailors, and later Britons in general, the nickname 'limeys'.

What is the unit sailors use to measure a ship's speed at sea?

A knot is one nautical mile (1.852 km) per hour; the name comes from counting the knots on a log line paid out behind a ship while a sandglass ran.

Which whale migrates each year between Arctic seas and Mexican lagoons?

Gray whale: the round trip of about 16,000 km is one of the longest migrations of any mammal, made so the whales can feed in the Arctic in summer and calve in the shallow lagoons of Baja California.

Which marine ecosystem is nicknamed the 'rainforest of the sea' for its biodiversity?

Coral reefs cover well under 1% of the ocean floor yet shelter about a quarter of all marine species, which is how they earned the comparison with rainforests.

Who in 2019 became the first person to dive to the deepest point of all five oceans?

Victor Vescovo finished the Five Deeps Expedition in 2019 in his submersible Limiting Factor; at the very bottom of the deepest trench of all he reported seeing what looked like a plastic bag.

Which sea lies between Italy's east coast and Croatia?

The Adriatic Sea is a long dead-end gulf, so when the sirocco blows up its length the water piles against the closed northern end and helps flood Venice, the 'acqua alta'.

Which species of crab has the largest leg span of any arthropod, over 3.5 metres from claw to claw?

Japanese spider crab, whose Japanese name taka-ashi-gani means 'tall-legged crab'. Adults live as deep as 600 metres and can weigh 19 kg, yet for all their fearsome looks they are reported to be gentle.

Which waxy substance, formed in the gut of whales and found floating at sea, was long prized by perfume makers?

Ambergris is thought to form around the sharp beaks of prey that a whale cannot digest; perfumers valued the lumps that wash ashore because they make a scent last far longer.

Why does clear, deep ocean water look blue?

Water absorbs red light more than blue: the reds are soaked up near the surface, so the light that returns from depth is blue, and red deep-sea animals look black down there.

Despite their name, horseshoe crabs are most closely related to which animals?

Spiders and scorpions: horseshoe crabs are not crustaceans at all, and one genetic study even placed them inside the arachnids; similar-looking fossils go back about 445 million years.

Horseshoe crab blood is blue because it carries oxygen with which metal?

Copper gives the blood its colour, and the blood clots at the faintest trace of bacterial toxins, which is why drug makers use it to check vaccines and intravenous drugs.

In fiddler crabs, which individuals have one claw dramatically larger than the other?

Only males carry the oversized claw: they wave it to attract females, who judge both its size and the quality of the wave, and wrestle rival males with it.

Why do hermit crabs live inside empty snail shells?

Their abdomen is soft and unarmoured, so a borrowed shell is their only armour; when a better one turns up, crabs can queue by size and each move into the next one's cast-off, a 'vacancy chain'.

Crabs, lobsters and shrimp belong to which order, named in Greek for their number of legs?

Decapods means 'ten-footed': crabs have four pairs of walking legs plus a pair of claws. The crab shape itself has evolved separately in at least five groups, a trend called carcinisation.

Which explorer named the Pacific 'Mar Pacífico', meaning peaceful sea, in 1520?

Ferdinand Magellan, who had fought through the strait now named for him and met favourable winds on the far side. He did not survive the circumnavigation; he was killed in the Philippines the next year.

Which imaginary boundary did Samoa jump across in 2011?

The International Date Line runs through the Pacific near 180° longitude; Samoa skipped 30 December 2011 altogether to move to its western side, in step with its main trade partners.

What is a 'red tide'?

A red tide is a bloom of toxic algae that discolours the water; dinoflagellates such as Karenia brevis off Florida release toxins that kill fish and taint shellfish.

Which seasonal wind system reverses major currents in the Indian Ocean twice a year?

Monsoon comes from the Arabic word for 'season': the winds blow one way in summer and the other in winter, and for centuries sailors timed their trading voyages across the Indian Ocean by them.

Which element's scarcity limits plankton growth across the Southern Ocean and equatorial Pacific?

Iron: in the Southern Ocean and equatorial Pacific nitrate and phosphate are plentiful yet plankton stay sparse, and iron-seeding experiments such as IronEx in 1993 triggered blooms.

More than five million years ago, which sea was cut off from the Atlantic and largely dried up?

The Mediterranean Sea lost much of its water to evaporation in the Messinian salinity crisis, leaving vast salt deposits under its floor, until the Atlantic is thought to have burst back in through Gibraltar in the Zanclean flood.

The Strait of Gibraltar separates Spain from which African country?

Morocco, though not all of the African shore is Moroccan: the port of Ceuta, facing Gibraltar across the strait, has been a Spanish city for centuries.

What carved the steep-sided coastal inlets known as fjords?

Glaciers carved fjords as deep U-shaped valleys that the sea flooded once the ice retreated; Sognefjord, Norway's longest and deepest, plunges more than a kilometre.

Over 90% of the deep water of which sea is anoxic, helping preserve ancient shipwrecks?

The Black Sea's deep layer never mixes with the oxygenated water above it, so no oxygen reaches the depths; its only outlet runs through the Bosporus towards the Aegean.

Which sea does the Kiel Canal join to the North Sea?

The Baltic Sea: the 98 km Kiel Canal across Schleswig-Holstein links it to the North Sea's German Bight, sparing ships the passage around Denmark.

The Bering Strait is named after an explorer of which nationality?

Vitus Bering, a Danish-born officer in the Russian navy, sailed through the strait in 1728; it is about 82 km wide at its narrowest.

Which strait runs between the Indonesian island of Sumatra and the Asian mainland?

Malacca: the strait is the shortest sea route between India and China, which has made it one of the most heavily travelled shipping channels in the world.

The Strait of Hormuz provides the only sea passage from the open ocean into which body of water?

The Persian Gulf has no other outlet to the open sea, so in the early 2020s about a quarter of the world's seaborne oil trade had to pass through the Strait of Hormuz, between Iran and Oman.

Which archipelago lies across a narrow strait from the southern tip of mainland South America?

Tierra del Fuego, Spanish for 'Land of Fire', was named for the many fires that the first European sailors to pass saw its native peoples burning along the shore.

Which warm current flows north along the west side of the North Pacific?

The Kuroshio takes its name from the Japanese words for 'black' and 'tide', after the deep blue of its water, and it carries tropical warmth north past Japan.

The cold Humboldt Current flows north along the west coast of which continent?

South America's Pacific coast is chilled by the Humboldt Current, which is largely responsible for the dryness of Chile's Atacama, one of the driest deserts on Earth.

What is the largest flow of seawater on Earth, also called the West Wind Drift?

The Antarctic Circumpolar Current meets no landmass to block it, so it loops right round the globe, moving more than 100 times the combined flow of all the world's rivers.

The Agulhas Current flows south along the east coast of which continent?

Africa's east coast is swept by the swift Agulhas Current; where storm swells run against it, freak waves rear up that have crippled or sunk large ships.

If Mount Everest stood in the deepest part of the Mariana Trench, about how much water would cover its summit?

About 2 kilometres: the trench floor lies nearly 11 km down and Everest stands about 8,850 m tall, so its summit would still be drowned; even so, shrimp-like amphipods live in the crushing dark at the bottom.

Which trench holds the deepest point in the Atlantic Ocean?

The Puerto Rico Trench plunges more than 8,300 metres at the Milwaukee Deep; it marks the boundary where the North American and Caribbean plates grind past each other.

The Sunda Trench, off Sumatra, is also known by the name of which neighbouring island?

Java gives the Sunda Trench its older name, the Java Trench; here the Australian plate dives beneath Indonesia, feeding the chain of volcanoes that runs along Sumatra and Java.

Whose hand-drawn 1950s maps of the Atlantic floor revealed a rift valley where the seabed is pulling apart?

Marie Tharp plotted ships' depth soundings by hand at Columbia and spotted the rift; her colleague Bruce Heezen first waved it off as 'girl talk', then published the Atlantic map with her in 1957.

In the 1930s William Beebe and Otis Barton dived half a mile down off Bermuda in which craft?

The Bathysphere was a hollow steel ball under 1.5 metres across, dangled on a cable from a ship; in 1934 it reached 923 metres as Beebe telephoned descriptions of the animals outside up to the deck.

Which French naval officer co-invented the Aqua-Lung diving set in 1943?

Jacques Cousteau teamed up with engineer Émile Gagnan, who adapted a valve built to feed gas to engines into a regulator that gives a diver air on demand; it let Cousteau film freely underwater.

The research submersible Alvin, commissioned in 1964, is operated by which organisation?

Woods Hole Oceanographic Institution has run Alvin for the US Navy since 1964; in 1966 the little sub found a hydrogen bomb lost in the sea off Spain, and in 1986 it took the first people down to the Titanic.

Which stratosphere balloonist later designed the bathyscaphe Trieste?

Auguste Piccard hung the Trieste's crew sphere beneath a float of petrol, which is lighter than water, just as his stratosphere gondola had hung beneath a balloon of gas.

Which trees form dense tropical coastal forests, standing on stilt-like roots in salt water?

Mangroves thrive where salt would kill most trees; their tangled prop roots slow the tides and trap mud, so the forests hold the shoreline together and blunt storm surges.

What is the biggest living species of fish?

The whale shark is a gentle giant: despite a mouth over a metre wide, it feeds by filtering tiny drifting animals and small fish from the water, and its rows of tiny teeth play no part in feeding.

Which animal has the largest brain on Earth, over five times heavier than a human's?

The sperm whale carries the largest brain known in any animal, living or extinct; it dives deep to hunt in total darkness, finding its prey with bursts of sonar clicks.

Which marine mammal holds the records for both the deepest and the longest dive?

Cuvier's beaked whale: a study published in 2014 logged a 2,992-metre dive off California, and one in 2020 a dive lasting 222 minutes; the whales go that deep to hunt squid.

Which marine mammal has the densest fur in the animal kingdom?

The sea otter's coat packs up to about 150,000 hairs per square centimetre; unlike most marine mammals it relies on fur rather than blubber, and it uses rocks as tools to open shells.

Sea otters are a keystone species because they control which animal that would otherwise devastate kelp forests?

Sea urchins: without otters to eat them, urchins graze kelp down to 'urchin barrens'; fur hunting between 1741 and 1911 cut otters to as few as 1,000-2,000 animals.

In which fish does the male carry the eggs in a pouch and give birth to the young?

The seahorse female lays her eggs in a pouch on the male's tail; he fertilises and nourishes them for weeks, then pumps out fully formed young with muscular contractions.

How many hearts does an octopus have?

Three: two hearts push blood through the gills and one sends it round the body, and that main heart stops beating whenever the octopus swims, which is why it tires fast and prefers to crawl.

Which group of fish has a skeleton made of cartilage rather than bone?

Sharks and rays are cartilaginous fish (class Chondrichthyes): their skeletons are cartilage rather than bone, which is why fossil sharks are known mostly from their teeth.

Which crustacean smashes snail shells with a club so fast that the water around it flashes into vapour?

The mantis shrimp's strike is among the fastest in the animal world; the vapour bubbles it creates collapse with a second shock, so even a punch that misses can stun or kill its prey.

A male narwhal's spiral tusk is actually what?

The tusk is a protruding left canine tooth that spirals through the upper lip and can reach 3 metres; medieval Europeans took such tusks for unicorn horns and gave them as gifts to kings and queens.

The orca is the largest member of which family?

Oceanic dolphins count the orca as their largest member; its pods are specialists, some eating only fish while others hunt seals and even other whales.

Which is the largest living sea turtle?

The leatherback is the only sea turtle without a hard shell: its back is tough, rubbery skin, and it can dive more than a kilometre down, deeper than most marine mammals.

Which 'living fossil' fish, known only from ancient rocks, turned up alive in a fishing trawler's net in 1938?

The coelacanth was spotted in a South African catch by museum curator Marjorie Courtenay-Latimer, who had it preserved; the genus was named Latimeria for her, a fish thought to have died out with the dinosaurs.

What is the longest bony fish alive, growing to about 8 metres?

The giant oarfish swims with a snaking, undulating motion, and its long, ribbon-like body may be the truth behind many sea serpent sightings.

Which cold-water jellyfish, among the largest known, can trail tentacles some 30 metres long?

The lion's mane jellyfish snares fish with more than a thousand stinging tentacles hung beneath its bell; the longest on record, from a giant found off Massachusetts in 1865, were put at about 36 metres.

Which seaweed, among Earth's fastest-growing organisms, can lengthen by about half a metre a day?

Giant kelp can add about half a metre in a day; gas-filled bladders at the base of each blade float it up toward the sunlight, building underwater forests tens of metres tall.

Charles Darwin's model explains atoll formation by the sinking of what?

A fringing reef around a volcanic island keeps growing upward as the island slowly subsides beneath the surface, leaving a ring of reef around a lagoon.

What do scientists call a lone wave more than twice as high as the surrounding sea?

A rogue wave towers over the swell around it; sailors told of such monsters for centuries, but scientists accepted them as real only once instruments began to record them.

The Draupner wave, a 25-metre giant recorded by a gas platform's laser on 1 January 1995, struck in which sea?

The North Sea platform logged a wave about twice the height of the 12-metre seas around it, the hard measurement that helped persuade sceptical scientists such freak waves were real.

The sea rushes up China's Qiantang River as a wall of water known as what?

Tidal bore: the rising tide is funnelled into a narrowing river mouth and piles up into a wave; the Qiantang's, nicknamed the Silver Dragon, is the world's largest, up to 9 metres high and racing upriver at up to 40 km/h.

World Oceans Day was first proposed in 1992 at the Earth Summit held in which city?

Rio de Janeiro's 1992 Earth Summit heard the idea first, but the day waited 16 years for official status: the UN General Assembly designated 8 June as World Oceans Day only in December 2008.

A country's exclusive economic zone extends how far from its coast?

200 nautical miles (370 km): inside that zone a coastal state holds the rights to the fish, oil and gas, yet the surface stays international water that foreign ships may cross freely.

The Great Blue Hole, a marine sinkhole 318 metres across, lies off which country?

Belize's Great Blue Hole formed as a dry cave during the ice ages, when sea level was far lower, which is why divers find stalactites deep inside; the rising sea later flooded it.

The wreck of the Titanic lies at a depth of roughly how many metres?

About 3,800 metres down, the wreck lies in two main pieces some 600 metres apart, proof that the liner broke up as she sank, as survivors had said and the 1912 inquiries had refused to accept.

What do sailors call the windless belt near the equator where sailing ships could sit becalmed for weeks?

The doldrums lie where the trade winds of the two hemispheres collide; heated by the equatorial sun, the air there rises straight up instead of blowing along the surface, leaving ships with nothing to fill their sails.

A whale fall on the deep seabed can sustain organisms for how long?

Decades: scavengers strip the flesh first, then bone-eating worms and bacteria that feed on sulphides from the decaying bones sustain a whole community for many years more.

What material are a whale's baleen plates made of?

Baleen is keratin, the horny protein of hair and fingernails; sold as 'whalebone', its springy strips once stiffened corsets, though it has nothing to do with bone.

Clownfish rely on which animals for shelter and protection from predators?

Sea anemones sting most fish that touch them, but a coat of mucus lets the clownfish nestle unharmed among the tentacles, safe from the predators that dare not follow.

Starfish tube feet are operated by what kind of system?

Hydraulic: a starfish pumps seawater, not blood, through its 'water vascular system' to work hundreds of tube feet, which grip prey while it pushes its stomach out of its mouth to digest the meal.

What do oceanographers call the endless shower of dead organisms and waste drifting down to feed the deep sea?

Marine snow can take weeks to settle to the bottom, and because nothing can photosynthesise in the dark, most deep-sea animals depend on these falling flakes for their food.

Which is the second-largest bay in the world after the Bay of Bengal?

Hudson Bay is named for Henry Hudson, who sailed into it in 1610; after a winter trapped in the ice his crew mutinied and set him adrift in a small boat, and he was never seen again.

The 2010 blowout of which drilling rig spilled oil into the Gulf of Mexico for 87 days?

Deepwater Horizon exploded on 20 April 2010, killing 11 workers; the well below it gushed for nearly three months, the largest offshore oil spill in US history, and cost BP more than $65 billion.

The Philippines officially calls the part of which sea inside its exclusive economic zone the West Philippine Sea?

The South China Sea name stays on world maps, but Manila began saying 'West Philippine Sea' in 2011 as its Spratly dispute with China sharpened, and a 2012 presidential order made it official.

In which sea did carrier fleets fight a 1942 battle without the opposing ships ever sighting each other?

The Coral Sea battle of May 1942 was fought entirely by carrier aircraft, the fleets never coming within sight of each other, and it turned back Japan's invasion force bound for Port Moresby.

The Denmark Strait cataract, an undersea overflow, lies between which two land masses?

The Denmark Strait, between Greenland and Iceland, hosts the world's largest waterfall: cold Nordic Seas water spills over the sill and cascades about 3,500 metres down, deflected by the Coriolis effect.

What does the Beaufort scale measure at sea?

Wind force is graded from 0 (calm) to 12 (hurricane) on the scale Royal Navy officer Francis Beaufort devised in 1805, so that sailors could judge the wind by eye, with no instruments at all.

Which Antarctic ice shelf, the continent's largest, calved iceberg B-15 in 2000?

The Ross Ice Shelf, itself about the size of France, shed B-15 in March 2000: a slab as big as Jamaica whose largest fragment later kept McMurdo Sound choked with sea ice and hampered supply ships to its research stations.

Which ray is the largest in the ocean, with a wingspan that can reach 7 metres?

The manta ray has the largest brain of any fish, and in a 2016 experiment captive mantas studied their own reflections in a mirror, behaviour associated with self-awareness.

Which vertebrate has the longest known lifespan, with one individual estimated to be about 400 years old?

The Greenland shark grows only about a centimetre a year in its icy water; radiocarbon dating of eye lenses in 2016 put one female at roughly 392 years, and the species may not breed until about 150.

The genus name Vampyroteuthis combines Latin 'vampyrus' with the Greek word for which animal?

Squid: Vampyroteuthis infernalis means 'vampire squid from hell', yet the animal is neither a vampire nor a true squid, and it feeds on drifting scraps of dead matter, not blood.

Hagfish defend themselves against predators by releasing what?

Slime: a threatened hagfish turns the water around it to thick gel within moments, choking an attacker's gills; it then ties its own body into a loop and slides it along to scrape itself clean.

Which species is considered the longest-living mammal, at over 200 years?

The bowhead whale: old harpoon points found in whales caught in modern times showed some had survived hunts more than a century before, and chemical dating of eye lenses put one animal at about 211 years.

Turritopsis dohrnii is nicknamed the immortal jellyfish because it can revert to which stage?

It can revert to the polyp stage: after maturing, the medusa's cells transform back into a sexually immature colonial polyp, a cycle that can in principle repeat indefinitely.

Though it looks like a jellyfish, the Portuguese man o' war is actually what?

A siphonophore: it is not one animal but a colony of genetically identical zooids, each shaped for a different job such as floating, feeding or reproduction.

Which marine mammal, discovered by Europeans in 1741, was hunted to extinction by 1768?

Steller's sea cow, a seaweed-grazing giant up to 9 metres long, floated in the shallows unable to dive, so hunters provisioning their ships with its meat wiped it out within 27 years of finding it.

Which seabird, with the longest wingspan of any living bird, can glide over the ocean for hours without a wingbeat?

The wandering albatross can spread its wings more than 3 metres and ride the wind for hours without flapping; some circle the entire Southern Ocean three times in a single year.

Which giant sea monster of Norwegian legend did Bishop Erik Pontoppidan describe in his 1750s natural history of Norway?

The Kraken, by Pontoppidan's account, was so vast that its back rose from the sea like a cluster of islands, and when it sank again it was said to leave a whirlpool that could engulf boats.

The word 'ocean' comes from Oceanus, who was what in Greek mythology?

The elder of the Titans in Hesiod's telling, Oceanus was pictured not as a sea but as a great river circling the whole Earth; his daughters were the sea nymphs called Oceanids.

Where is most of Earth's fresh water held?

In ice: about 97% of Earth's water is salty, and roughly two-thirds of the fresh remainder is frozen in ice sheets and ice caps; most of the rest lies underground, and lakes and rivers hold only a sliver.

Deep-sea hydrothermal vent ecosystems were first directly observed in 1977 near which islands?

Geologists diving in Alvin at the Galápagos Rift in 1977 found the vents teeming with life, a shock since no sunlight reaches them and the animals depend on chemosynthetic microbes.

What nickname do scientists give seafloor chimneys that belch scalding fluid thick with iron sulfide particles?

Black smokers get their colour from iron sulfide, which is black; it crystallises the instant the scalding vent fluid meets near-freezing seawater, and the fallout builds the chimneys themselves.

Fertiliser runoff from Midwest farms flows down which river to feed the Gulf of Mexico's summer dead zone?

The Mississippi delivers nitrogen and phosphorus that fuel vast algal blooms in the Gulf; when the algae die and rot they use up the oxygen in the bottom water, and fish must flee or suffocate.

The 1992 collapse of which fish on Newfoundland's Grand Banks ended a fishery nearly five centuries old?

Cod had fed Newfoundland for nearly five centuries, but by 1992 the breeding stock had crashed to about 1% of its peak; the fishing ban that followed put tens of thousands of people in the province out of work.

Which nickname is given to the ocean's bathypelagic layer, which begins about 1,000 metres down?

The Midnight zone runs from about 1,000 to 4,000 metres down, and the only light there comes from animals that make their own.

Away from volcanic vents, deep ocean water stays at roughly what temperature all year round?

Near freezing is the rule: the deep ocean averages only about 4 °C, because cold water is denser and sinks beneath the warm surface layer.

Named for its ear-like fins, the Dumbo octopus holds which record among the ocean's octopuses?

Deepest living of all known octopuses, Dumbos have been filmed nearly 7,000 metres down in an ocean trench, far below the reach of sunlight.

The open ocean's sunlit top layer, where photosynthesis is possible, ends at roughly what depth?

200 m is about as far as useful sunlight reaches in clear open water, yet this thin layer is home to roughly ninety percent of marine life.

Pressure in the ocean's deepest trenches is roughly how many times the air pressure at sea level?

More than 1,000 times surface pressure bears down on the trench floors, because every 10 metres of seawater adds roughly one more atmosphere.

Countless ocean animals leave the dim middle depths to feed near the surface at what time of day?

Around dusk the climb begins, and by biomass this daily commute is the largest synchronised migration on Earth, reversed again before sunrise.

Bacteria feeding on sinking debris leave the ocean's middle depths short of which dissolved gas?

Oxygen runs so low there that the layer is known as the oxygen minimum zone, because microbes use the gas up faster than currents can replace it.

The ocean's abyssal zone takes its name from the Greek word abyssos, which means what?

Bottomless is an exaggeration, but only just: the abyssal zone lies 4,000 to 6,000 metres down and remains in perpetual darkness.

Which term describes life on the ocean floor itself, as opposed to in the open water above?

Benthic comes from a Greek word for the depths of the sea, and the bottom dwellers living there are collectively known as the benthos.

What makes up the ocean's 'deep scattering layer', which wartime sonar crews mistook for the seabed?

Schools of small fish, chiefly lanternfish, bounce sonar off their gas-filled swim bladders, an echo so solid that it became known as the false bottom.

Which deep-ocean relative of the garden woodlouse can curl itself into an armoured ball?

Giant isopod species can reach about 50 centimetres, a textbook case of deep-sea gigantism, yet they look much like their tiny cousins on land.

Which soft-bodied relatives of starfish often make up most of the animal biomass on the deep ocean floor?

Sea cucumbers account for about 90 percent of the larger animal life below 8,900 metres, and some jelly-like species can lift off the mud and swim.

Which deep-ocean fish looks up through its own transparent, dome-shaped head to spot prey overhead?

Barreleye fish have tubular eyes that usually point straight up at the outlines of prey, yet they can swivel them forward when it is time to eat.

Which term covers ocean swimmers, such as tuna and whales, strong enough to move against currents?

Nekton was coined by the German biologist Ernst Haeckel to separate active swimmers from the drifters that are carried wherever the water takes them.

Which term describes the pitch-dark bulk of the ocean, where no sunlight penetrates at all?

Aphotic comes from the Greek for 'without light', and in these depths glowing animals supply more light than the Sun ever does.

Which shallow ocean zone stretches from the low-tide mark out to the edge of the continental shelf?

Neritic waters are sunlit all the way to the seabed, and the food web that grows there supports most of the world's great wild fisheries.

Which finger-sized deep-ocean fish are thought to be the most numerous vertebrates on Earth?

Bristlemouths number in the hundreds of trillions to quadrillions, and each one carries rows of light organs along its underside.

Which term describes ocean fish, such as cod and sole, that live and feed on or near the seabed?

Demersal fish are also known as groundfish, and the flatfish among them lie on the bottom with both eyes facing upwards.

Which layer of the ocean traps low-pitched sound so well that it can carry for thousands of miles?

SOFAR channel is short for sound fixing and ranging; sound travels slowest at that depth, so it bends back into the layer rather than leaking away.

Which human activity is the chief source of the gas that is steadily acidifying the ocean?

Burning fossil fuels releases the gas into the air, and the sea has soaked up nearly a third of everything people have emitted.

Sharing the same root cause, ocean acidification has been called the 'evil twin' of which problem?

Global warming and acidification stem from the same emissions, but acidification is a matter of chemistry in the water rather than heat in the air.

Ocean acidification, which weakens shells and skeletons, is nicknamed after which bone disease?

Osteoporosis of the sea is the phrase, because more acidic water eats away at the minerals that clams, lobsters and shrimp use to build their hard parts.

On the pH scale used to track ocean acidification, which value marks a perfectly neutral liquid?

7 is neutral, the pH of pure water; anything lower is acidic and anything higher, seawater included, is basic.

Ocean surface water has become about 30 percent more acidic since the start of which historical era?

Industrial Revolution factories began the change more than 200 years ago, and because the pH scale is logarithmic a small-looking drop means a large jump in acidity.

Seawater becomes more acidic as it gains more of which ions, the ones that the pH scale measures?

Hydrogen ions are what the pH scale counts, and one whole unit lower on the scale means ten times as many of them.

Ocean acidification makes it harder for clams and sea snails to build shells from which compound?

Calcium carbonate is built from carbonate ions, and acidifying seawater leaves fewer of them available to the animals that need them.

Which pea-sized snails, nicknamed sea butterflies, have shells that dissolve as the ocean acidifies?

Pteropods placed in seawater as acidic as that projected for 2100 saw their shells slowly dissolve within 45 days, a worry because krill, fish and whales all eat them.

Siberian eruptions acidified the oceans during which mass extinction, the worst in Earth's history?

End-Permian seas lost about 81 percent of their species in what is nicknamed the Great Dying, and animals with chalky skeletons fared worst of all.

Ocean acidification may threaten which tiny algae, whose chalky plates built the White Cliffs of Dover?

Coccolithophores armour themselves in tiny chalk discs, and countless discs settling on ancient seabeds formed the white rock of the cliffs.

Which far-northern predator is officially classed as a marine mammal because it depends on sea ice?

Polar bear life is tied to the frozen sea, the platform for hunting seals; when the ice melts in summer the bears are forced onto land until it returns.

Which salt-tolerant trees line tropical coasts, their tangled roots standing in seawater?

Mangroves filter the salt out of seawater, and their sheltered root tangle doubles as a nursery where young lemon sharks practise hunting.

What name is given to the calm, shallow water enclosed by the ring of an atoll or by a barrier reef?

Lagoon comes from the Italian laguna and was first used in English for the waters around Venice; the word goes back to the Latin lacus, a lake or pond.

Which colourful reef fish grind up coral while grazing and excrete it as fine white sand?

Parrotfish scrape algae off rocks and dead coral, and one large fish can excrete hundreds of pounds of white sand a year, the stuff of Hawaii's famous beaches.

Which reef-building shellfish can each filter up to 50 gallons of seawater in a single day?

Oysters fuse together as they grow into rock-like reefs, and by filtering out algae they clean water that is overloaded with nutrients.

Which coastal wetland, flooded by the tides, is dominated by grasses and herbs rather than trees?

Salt marsh grasses slow the water so that mud settles around their stems, and birds raise their young in the tall cover.

Which ecosystems are formed by flowering plants whose ancestors returned from land to the ocean?

Seagrass meadows descend from land plants that recolonised the sea 70 to 100 million years ago, and they now cover only about 0.1 to 0.2 percent of its surface.

Bacteria at deep-ocean vents turn sulfides rather than sunlight into food through which process?

Chemosynthesis supports whole communities at vents, cold seeps and even whale carcasses, with microbes rather than plants at the base of the food chain.

Which marine region around Indonesia and the Philippines is nicknamed the 'Amazon of the seas'?

Coral Triangle waters hold more than three-quarters of the world's reef-building species and six of the seven kinds of sea turtle.

Which term describes the nutrient overload, often from farm fertiliser, behind coastal dead zones?

Eutrophication feeds huge blooms of algae, and when they die the bacteria that rot them use up the gas that fish need to breathe.

What is the lowest point of an ocean wave, the dip that lies between two crests, called?

Trough is the opposite of crest, and oceanographers measure a wave's height as the vertical distance between the two.

Swimmers are told to swim parallel to the shore to escape which narrow, fast flow at surf beaches?

Rip current speeds can reach eight feet per second, faster than an Olympic swimmer, so fighting straight back to the beach only brings exhaustion.

What term describes the horizontal distance from one ocean wave's crest to the crest of the next?

Wavelength at sea ranges from a few centimetres for wind ripples to hundreds of metres for the longest storm waves.

What is the abnormal rise in sea level called when a hurricane's winds push water onto the coast?

Storm surge, not wind, is the greatest threat to life in a hurricane, as the water piles up on top of the normal tide.

The Japanese word tsunami, adopted by ocean scientists worldwide, literally means what in English?

Harbour wave is the literal meaning, and scientists prefer the Japanese word to 'tidal wave' because these surges have nothing to do with the tides.

What name is given to smooth, regular ocean waves that have outrun the storm that created them?

Swell can cross whole ocean basins: the breakers on a beach were often born in storms thousands of nautical miles away.

An undersea canyon funnels record-breaking surf onto the beach of which Portuguese fishing town?

Nazaré sits at the head of Europe's largest submarine canyon, about 5,000 metres deep, which focuses incoming waves into faces more than 20 metres high.

Which tides, the month's weakest, arrive around the first and third quarters of the lunar cycle?

Neap tides come twice a month, about seven days after the biggest ones, when the two bodies that raise the sea partly cancel each other out.

What do sailors call the brief pause when a tidal stream stops flowing before it reverses direction?

Slack water rarely lines up exactly with high or low tide; in some narrow channels the stream keeps running for hours after the level turns.

What do oceanographers call the stretch of open water over which wind blows to build waves?

Fetch, wind speed and how long the wind keeps blowing together decide how big the waves get, which is why small lakes never see ocean-sized breakers.

What do sailors call the outgoing phase of the tide, when seawater drains away from the shore?

Ebb is the falling half of the cycle, the hours that uncover the intertidal zone, while the incoming half is known as the flood.