149 Fun Facts About Oceans

Which is the smallest and shallowest of Earth's five oceans?

The Arctic Ocean is mostly covered by sea ice, which reflects far more of the Sun's energy than open water, so as the ice melts, the darker water absorbs more heat and speeds the melting.

In which sea creature does the male, rather than the female, carry the young and give birth?

Seahorses: the female places her eggs in the male's brood pouch, and he carries them for weeks before pushing the young out with muscular contractions.

Ocean phytoplankton produce about half of Earth's oxygen through which process?

Photosynthesis: because phytoplankton need light to make food, they drift in the ocean's top layer, and they form the base of most marine food webs.

Which sea creature's internal shell gives pet birds extra calcium?

Cuttlefish: the cuttlebone is divided into chambers, and the animal changes their mix of gas and liquid to rise or sink.

Which whale migrates each year between icy northern feeding grounds and calving lagoons in Mexico's Baja California?

Gray whales make one of the longest annual migrations of any mammal, and after whaling drove them to near extinction, the eastern population recovered enough to leave the US endangered list.

Which fish makes its home among the stinging tentacles of a sea anemone?

Clownfish: a mucus coating stops the anemone from stinging them, and in return they chase off anemone-eating fish and feed their host with their waste.

Which process brings cold, nutrient-rich deep water to the ocean surface, creating rich fishing grounds?

Upwelling: winds push surface water aside and deeper water rises to replace it, so when El Niño weakens those winds, fewer nutrients rise and fish stocks suffer.

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

Coral reefs cover under 1% of the ocean yet shelter at least a quarter of marine species, and when water gets too warm, corals expel their food-making algae and bleach white.

Which US Navy lieutenant reached the Challenger Deep with Jacques Piccard in 1960?

Don Walsh: past 9,000 metres one of Trieste's outer windows cracked with a bang that shook the craft, but the pair kept going and spent about 20 minutes on the bottom.

Which 'sea', really a salt lake, is bordered by Jordan, Israel and the West Bank?

The Dead Sea is about 9.6 times saltier than the ocean, so swimmers float with ease, while fish and plants can't live in it at all.

Which effect deflects ocean currents to the right in the Northern Hemisphere?

The Coriolis effect comes from Earth's rotation and steers winds and ocean currents into huge swirls, yet it is far too weak to decide which way a sink or toilet drains.

Which undersea mountain range surfaces at Iceland, where two tectonic plates pull apart?

The Mid-Atlantic Ridge crosses Iceland at Þingvellir National Park, where divers in the Silfra fissure swim in a water-filled rift between the North American and Eurasian plates.

Which raft did Thor Heyerdahl sail across the Pacific in 1947?

Kon-Tiki: Heyerdahl set out to show South Americans could have settled Polynesia, but the evidence points to settlers from Island Southeast Asia instead.

Balboa sighted the Pacific in 1513, but which explorer gave it its name in 1520?

Ferdinand Magellan called it Mar Pacífico, the 'peaceful sea', but his fleet then spent three months and twenty days crossing it, and many of the crew died of scurvy.

What mainly causes the twice-daily rise and fall of Earth's ocean tides?

The Moon's gravity drives the tides, and when the Sun lines up with it at new and full moon, the extra pull makes 'spring tides', a name with nothing to do with the season.

Which scale rates wind at sea from calm to hurricane force?

Beaufort: Admiral Francis Beaufort devised the scale in 1805 so sailors could estimate wind strength from what they saw.

The Atlantic's deepest point lies in a trench named after which Caribbean island?

Puerto Rico: the trench runs just north of the island, and in 1918 an earthquake off its northwest coast sent a tsunami onto its west coast, killing dozens.

Which is the warmest of the world's oceans?

The Indian Ocean is the world's warmest, and Asia's landmass to its north drives its monsoon, with winds blowing from the southwest in summer and from the northeast in winter.

Which whale's songs filled a best-selling 1970 album produced by Roger Payne?

Humpback whales: Songs of the Humpback Whale helped build momentum for the Save the Whales movement against commercial whaling.

Which marine mammal often wraps itself in kelp to keep from drifting away while it rests?

Sea otters eat the sea urchins that graze on kelp, so coasts that lose their otters can turn into urchin barrens with no kelp forest left.

Which fish hatches in a river, grows up at sea, then swims back to its home stream to spawn?

Salmon find their way back to the stream where they hatched largely by smell, and every Pacific species dies within days or weeks of spawning.

Which explorer's 1770s voyages proved that water encircled the globe's southern latitudes?

James Cook's second voyage sailed deep into polar southern waters and over most of the predicted sites of Terra Australis, proving no such continent lay in temperate latitudes.

The Mariana Trench is named after islands whose largest is which US territory?

Guam: Spain named the islands Las Marianas in 1668 to honour Queen Mariana of Austria, then ceded Guam to the United States after losing the Spanish–American War.

Water pressure at the bottom of the Mariana Trench is roughly how many times that at sea level?

About 1,000 times: the deepest-living fish, found near 8,000 metres, carry high levels of a molecule called TMAO, which stops their proteins being squashed out of shape.

Which sea creature's blue blood is harvested to test injectable drugs and medical devices for bacterial toxins?

Horseshoe crabs: despite the name, they aren't true crabs at all, and they are more closely related to spiders and scorpions.

Which sea creature has three hearts and blue blood?

Octopuses: two hearts pump blood through the gills and one through the body, and the blood turns blue because it carries oxygen on a copper-based protein rather than iron.

Divers on the Great Barrier Reef cull which coral-eating starfish by injecting it with vinegar or bile salts?

Crown-of-thorns starfish: during an outbreak they can strip a reef of most of its living coral.

The Sargasso Sea is unusual among seas because it is bounded by what rather than land?

Four ocean currents, led by the Gulf Stream, swirl around it, so floating sargassum seaweed gathers in its calm centre, and so does a heavy load of plastic waste.

Which fish, whose breeding was a mystery since Aristotle, did Johannes Schmidt trace to the Sargasso Sea?

The European eel: Schmidt found ever-smaller larvae the farther he sailed toward the Sargasso Sea, yet no one has ever watched these eels spawn in the wild.

Which American polymath charted the Gulf Stream?

Benjamin Franklin noticed that mail ships from England took longer to reach America, so he charted the current with whaling captain Timothy Folger.

Which explorer's 1513 expedition gave Europeans their first record of the Gulf Stream?

Juan Ponce de León's April 1513 log off Florida records a current so strong that his ships were pushed backwards despite a fair wind.

What is a hurricane called when it forms in the northwestern Pacific Ocean?

Typhoon: hurricanes, typhoons and tropical cyclones are the same kind of storm, named by region, and all draw their energy from warm ocean water evaporating.

Which compound makes up most of seawater's dissolved minerals?

Sodium chloride: much of the ocean's salt comes from rocks on land, worn away by slightly acidic rain and carried to the sea by rivers.

Which sea, where intense heat drives fierce evaporation, is one of the world's saltiest?

The Red Sea's salinity climbs from about 36 parts per thousand in the south to about 41 in the north, because scant rainfall does little to replace the water lost to evaporation.

What decides whether sea turtle eggs hatch male or female?

Nest temperature: warmer sand produces more female hatchlings, so hotter nesting beaches can tip a population toward females.

Which enzyme powers the light-making reaction in most glowing deep-sea animals?

Luciferase speeds a reaction in which the molecule luciferin combines with oxygen and releases energy as light, so glowing animals need oxygen to shine.

The 'deep sea' is broadly defined as beginning at what depth, where light begins to fade?

About 200 metres: most light is absorbed in that top layer, so below it the water fades to twilight, and below about 1,000 metres no light penetrates at all.

What is the falling organic debris that feeds many deep-sea creatures called?

Marine snow carries carbon from the surface down to the deep, and whatever isn't eaten on the way settles into the muddy ooze blanketing the seafloor.

Hydrothermal vent water can top 400 °C; what keeps it from boiling?

Great pressure from the ocean above keeps the fluid liquid, and when it meets near-freezing seawater, its dissolved minerals drop out and build tall chimneys.

Which stinging sea creature is really a colony of specialised organisms rather than a single animal?

Man o' war: it is a siphonophore, and its gas-filled float somewhat resembles an old warship at full sail, which gave it its name.

Which remote spot in the South Pacific, farthest from any land, is used as a graveyard for old spacecraft?

Point Nemo: space agencies steer old spacecraft down there because it is so remote, with so little shipping, that falling debris is unlikely to hurt anyone.

The word tsunami literally translates as what?

Harbour wave: in deep water a tsunami barely raises the surface, so fishermen out at sea could miss it and return to find their harbour village devastated.

The bathyscaphe Trieste stayed buoyant with a huge float full of what, chosen because it barely compresses?

Gasoline: to rise again, the crew released iron-shot ballast held by electromagnets, which would drop the shot automatically if the power failed.

Which gas, absorbed by the ocean from the air, is making seawater more acidic?

Carbon dioxide: as seawater absorbs it, carbonate ions dwindle, making it harder for oysters, clams and sea urchins to build their shells.

What is a 'red tide' actually caused by?

A bloom of algae: some produce toxins that kill fish and make shellfish dangerous to eat, and despite the name, blooms vary in colour and have nothing to do with tides.

What did sailors call the windless belt near the equator where ships could be becalmed for weeks?

The doldrums: near the equator warm air mostly rises instead of blowing sideways, which leaves little wind at the surface to fill a ship's sails.

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

Storm surge: rising seawater, not wind, is the leading cause of deaths from hurricanes, because it floods low-lying coasts.

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

Hades, god of the underworld: most of the hadal zone lies in trenches, which form where one tectonic plate slides down beneath another.

In Coleridge's poem The Rime of the Ancient Mariner, shooting which seabird brings a curse on the ship?

Albatross: the crew hang the dead bird around the mariner's neck, which is why a burden of guilt is still called 'an albatross around one's neck'.

What is the partly enclosed coastal body of water where river water mixes with the sea called?

An estuary's mixing brings in high levels of nutrients, which makes estuaries among the most productive habitats on Earth, and many animals breed there.

Which tiny crustaceans swarm in the Southern Ocean and feed whales, seals and penguins?

Krill: when food runs short they can moult into a smaller body and shrink, an unusual trick for animals their size.

The Bay of Fundy, home to the world's highest tides, lies in which country?

Canada: the funnel-shaped bay resonates with the incoming tide, reinforcing each surge, which is how it builds the world's highest tidal range.

The 1995 Draupner wave, a 25.6-metre rogue wave, struck a Norwegian gas platform in which sea?

North Sea: the wave was more than twice as tall as its neighbours, and measuring it helped convince scientists that sailors' tales of freak waves were real.

Which ship of Ernest Shackleton's expedition was crushed by pack ice in 1915, stranding its crew on the ice?

Endurance: all 28 men survived after Shackleton crossed open ocean in a lifeboat to South Georgia to fetch help, and the wreck was finally found in 2022.

Which film director made a solo dive to the Mariana Trench's deepest point on 26 March 2012?

James Cameron later donated his sub, which he had helped design and field-test, to the Woods Hole Oceanographic Institution.

Which fish lies on the seabed with both eyes on one side of its head?

Flounder: young flounders swim upright with an eye on each side, then one eye migrates across the head as they settle on the bottom.

William Beebe and Otis Barton reached a record 923 m off Bermuda in 1934 in which vessel?

The Bathysphere hung from a steel cable, and a phone line let Beebe relay what he saw to Gloria Hollister, who recorded it at the surface.

Which submersible carried Robert Ballard to the Titanic wreck in 1986?

Alvin also located a hydrogen bomb lost in the sea off Palomares, Spain, after two US Air Force planes collided in 1966.

The 1977 discovery of vent life overturned the idea that all life depends on what?

Sunlight: vent tubeworms and mussels live instead on microbes that turn hydrogen sulphide from the vent fluid into food.

What is the largest living species of fish?

The whale shark is a slow filter-feeder that scoops up plankton and small fish, and despite its enormous size it poses no danger to swimmers.

What is the heaviest animal known to have ever lived?

The blue whale was hunted so heavily in the early 1900s that only a small fraction of its former numbers survives, yet its calls rank among the loudest of any animal.

Researchers from which country took the first photos of a live giant squid in its habitat, in 2004?

Japan: Tsunemi Kubodera and Kyoichi Mori lowered baited cameras about 900 metres off the Ogasawara Islands, and the snagged squid struggled for over four hours before tearing free.

Which 'living fossil' fish turned up in a fisherman's catch off South Africa in December 1938?

The coelacanth was thought to have died out about 66 million years ago, and its genus, Latimeria, honours museum curator Marjorie Courtenay-Latimer, who spotted it in the catch.

Which fish hitches rides on bigger animals using a sucker on top of its head?

Remora: the sucker is really a reshaped dorsal fin, whose slats open and close to grip sharks, rays and turtles.

Which oceanographer is nicknamed 'Her Deepness'?

Sylvia Earle: in 1979 she walked on the seafloor off Oahu at 381 metres, sealed inside an armoured JIM diving suit.

What was the name of Jacques Cousteau's famous research ship?

Calypso: Loel Guinness bought the ship in 1950 and leased it to Cousteau for a symbolic one franc a year.

Which crab protects its soft body by living inside empty snail shells, moving to bigger ones as it grows?

Hermit crabs: when a roomy shell turns up, crabs can line up from biggest to smallest, and each moves into the next one's hand-me-down home.

The Drake Passage separates the South Shetland Islands from which famous headland?

Cape Horn: its winds, currents and icebergs made it perhaps the most dangerous ship passage in the world, and many sailors died trying to round it.

Which sea mammal's long spiral tusk was sold in medieval Europe as the horn of a unicorn?

Narwhal: the 'horn' is really an overgrown tooth, usually just one on a male, spiralling out through the upper jaw.

The Bering Strait separates Alaska from which country?

Russia: Big Diomede, which is Russian, and Little Diomede, which is American, sit in the strait 3.8 km apart but on opposite sides of the International Date Line.

The Strait of Gibraltar, narrowing to about 14 km, separates which two continents?

Europe and Africa: lighter Atlantic water flows in through the strait at the surface, while denser, saltier water flows out beneath it along the bottom.

Sailors measured the angle of the Sun above the horizon to find their latitude with which instrument?

Sextant: latitude was the easier half, and longitude stayed a dangerous problem at sea until John Harrison's marine chronometer let navigators calculate it.

Which sea nearly dried up about six million years ago, leaving huge salt deposits in its basin?

The Mediterranean Sea: the Atlantic later poured back in through the Strait of Gibraltar and refilled it, perhaps in a sudden, colossal flood.

Ancient shipwrecks survive intact in which sea thanks to a permanent, oxygen-free deep layer?

The Black Sea: over 90% of its deep water holds no oxygen, because bacteria breaking down sinking dead matter use it all up.

Which sea is one of the world's largest bodies of brackish water?

The Baltic Sea: rivers pour in so much fresh water, and shallow sills let so little seawater in through the Danish straits, that it stays only mildly salty.

Which supercontinent was ringed by a single superocean about 250 million years ago?

Pangaea: Alfred Wegener proposed that the continents were once joined, but geologists rejected the idea until plate tectonics confirmed it after his death.

The world's widest continental shelf, stretching 1,500 km, lies off which coast?

Siberia: much of this shallow shelf was dry land during the last ice age, when sea level was more than 100 metres lower than today.

Fishermen off South America named the El Niño climate pattern after whom?

The Christ Child: the warm current tended to peak around Christmas, so they called it El Niño de Navidad, and its cold counterpart later became La Niña, 'the little girl'.

The Great Blue Hole, a giant marine sinkhole, lies off the coast of which country?

Belize: the Great Blue Hole formed as a limestone cave during ice ages, when sea level was far lower, then flooded as the seas rose, leaving stalactites deep inside.

A tiny male deep-sea anglerfish fusing permanently to a female is an example of what?

Sexual parasitism: the fused male's tissues and blood vessels join the female's and he becomes little more than a sperm supply, a natural case of parabiosis.

Which deep-sea cephalopod, when threatened, turns its webbed arms inside out over its body?

The vampire squid is neither a vampire nor a true squid but the last survivor of an ancient group of cephalopods, and it thrives where oxygen is scarce.

Which sea turtle is the largest turtle alive today?

Leatherback: it feeds on soft prey such as jellyfish, so it can mistake floating plastic bags for food and swallow them.

Which shark may live for centuries, longer than any other known vertebrate?

Greenland shark: the lens of its eye keeps growing through life, so radiocarbon dating the lens centre showed one was about 392 years old, give or take 120.

Which seabird makes the longest known migration of any animal?

Arctic terns fly from their northern breeding grounds to the far south and back each year, so they see two summers and more daylight than any other creature.

Which salt-tolerant coastal trees stand on tangled prop roots that make them look as if they're on stilts?

Mangroves: their dense root systems soak up wave energy, so mangrove forests shield coastlines from erosion and battering waves.

Which tanker ran aground in Alaska's Prince William Sound in 1989, spilling millions of gallons of crude oil?

Exxon Valdez: the disaster led Congress to pass the Oil Pollution Act of 1990, which phased in double-hulled tankers to add a barrier between the oil and the sea.

The 1820 sinking of the whaleship Essex by a sperm whale inspired which famous novel?

Moby-Dick found little audience when it came out in 1851, and only after the 1919 Melville revival was it hailed as one of the Great American Novels.

Which penguin's males balance the egg on their feet through the Antarctic winter?

Emperor penguin: the males huddle together, and birds on the cold outer edge slowly shuffle around so each one takes a turn inside.

Which shore animal lives glued head-down to rocks, sweeping in food with its legs?

Barnacle: though it wears a hard shell and never moves, it is a crustacean, related to crabs and lobsters.

Cold seeps were first discovered in 1983 on the Florida Escarpment in which body of water?

Gulf of Mexico: the tubeworms and mussels found there thrive on seeping brine, drawing their energy from chemicals such as methane and hydrogen sulphide.

Which warm current keeps the Arctic port of Murmansk free of ice all year?

North Atlantic Current: because its warmth kept Murmansk open in winter, Allied Arctic convoys shipped tanks, aircraft and food there during World War II.

The Kuroshio, a warm current in the western North Pacific, has a name meaning what?

Black Stream: kuro means black and shio means tide or current, a name the current earned from the deep, dark blue of its water.

The cold Humboldt Current off South America is also named after which country?

Peru: the cold current chills the coastal climate, and it is a big reason the Atacama is the driest non-polar desert on Earth.

A sunken whale carcass can sustain a deep-sea ecosystem for roughly how long?

Decades: after scavengers strip the flesh, bone-eating Osedax worms dissolve the skeleton with acid to reach the fats locked inside.

What are the black sand beaches of Hawaii mostly made of?

Volcanic rock: when lava pours into the sea, it cools so fast that it shatters into fragments of basalt, the black grains of the sand.

The deep Kermadec Trench lies north-east of which country?

New Zealand: the trench runs past New Zealand's Kermadec Islands, where the Pacific Plate dives beneath the Indo-Australian Plate.

Which technology did physicist Paul Langevin develop during World War I to hunt submarines?

Sonar: Langevin's detector timed how long a sound pulse took to echo back, but the war ended before his system became operational.

Which sea phrase titled Kipling's 1896 poetry collection?

The Seven Seas: the phrase is thousands of years older than Kipling, and which seven seas it meant changed from culture to culture.

Which sea creature's punch can crack aquarium glass?

Mantis shrimp: its club strikes so fast that it forms bubbles in the water, and when they collapse they hit the prey with extra force.

Which Hawaiian volcano is the tallest mountain on Earth when measured from its base on the Pacific seabed?

Mauna Kea rises more than 10,210 metres from base to peak, over a kilometre more than Everest stands above sea level. More than half of the mountain is hidden underwater.

Which precious metal is dissolved throughout the sea, at roughly one gram per 100 million tonnes of water?

Gold is spread so thinly through seawater that nobody has found a way to extract it at a profit. The chemist Fritz Haber spent years in the 1920s trying, then gave up.

The Atlantic Ocean is named after which Titan of Greek myth, who also gives his name to Morocco's mountains?

Atlas was the Titan made to hold up the heavens, and Greek writers called the waters past Gibraltar the 'Sea of Atlas'. Books of maps carry his name as well.

Unlike the South Pole, which sits on a continent, the North Pole lies on top of what?

Drifting sea ice covers the North Pole, floating on water more than 4,000 metres deep. The ice never stops moving, which makes a permanent base there impractical.

Which body of water, fed by the River Jordan, is the lowest freshwater lake on Earth?

The Sea of Galilee lies roughly 210 metres below sea level. Despite the name it is a lake, about 21 km long, and its water is fresh rather than salty.

Which busy stretch of sea do the French call La Manche, 'the sleeve', a name usually credited to its shape?

The English Channel tapers from about 240 km across at its widest to just 34 km at the Strait of Dover. The French name has been in use since at least the 17th century.

Nearly half the mass of the Great Pacific Garbage Patch, between Hawaii and California, is what kind of debris?

Fishing nets made of synthetic fibre account for nearly half the patch's mass, a 2018 study found. By number, most pieces are microplastics smaller than a fingernail.

Which river sends more fresh water into the Atlantic than any other river delivers to any sea?

The Amazon delivers about a fifth of all the river water that reaches the world's seas. Its flow is greater than that of the next seven largest rivers combined.

Ships have crossed between the Atlantic and Pacific since 1914 by way of a canal through which country?

Panama's canal lifts ships 26 metres by locks to an artificial lake, then lowers them at the far side. The lake was made by damming the Chagres River to cut down on digging.

What name is given to the belt around the Pacific where about 90% of the world's earthquakes strike?

The Ring of Fire also holds around two-thirds of the world's active or dormant volcanoes. Geologists know it as the Circum-Pacific belt.

True caviar traditionally comes from wild sturgeon of which body of water, the largest lake on Earth by area?

The Caspian Sea is bounded by five countries and covers an area larger than Germany. It accounts for 40–44% of all the lake water in the world.

Which sea, reached through the narrow Strait of Kerch, is the shallowest in the world, no more than 14 metres deep?

The Sea of Azov averages only about 7 metres deep. In its bays, where river silt has built up, the water is often barely a metre deep.

Which sea, bounded by Japan, Taiwan and the Mariana Islands, is often ranked the largest in the world by area?

The Philippine Sea covers about 5 million square kilometres of the western Pacific. In 1944 it was the scene of the largest carrier-against-carrier battle in history.

Which sea, known in Vietnam as the East Sea, contains the disputed Spratly and Paracel island groups?

The South China Sea holds hundreds of small islands, islets and shoals, and several countries make competing claims to them. Even the names used for the sea differ from shore to shore.

Which sea, nicknamed 'the Ditch' by people on both shores, honours the Dutch explorer who crossed it in 1642?

The Tasman Sea is about 2,000 km across. Abel Tasman also gave his name to Tasmania, which he reached on the same 1642 voyage and called Van Diemen's Land.

Which strait cuts Istanbul in two, leaving part of Turkey's largest metropolis on each of two continents?

The Bosporus is the world's narrowest strait in use as a shipping route between nations. At its tightest the two shores are only about 700 metres apart.

Which narrow waterway between Iran and Oman is the only sea route out of the oil-rich Persian Gulf?

The Strait of Hormuz is one of the world's most strategically important choke points. Tankers pass through in shipping lanes just two nautical miles wide.

Which busy waterway between the Malay Peninsula and Sumatra narrows to under 3 km wide near Singapore?

The Strait of Malacca is about 900 km long and is the main shipping channel between two oceans. Its heavy traffic has long made piracy and smuggling a concern.

Which Antarctic sea, named after a British seal hunter, has been judged to have the clearest water of any sea?

The Weddell Sea was first named after King George IV by its discoverer, who sailed into it in 1823. It was renamed in the sailor's own honour only in 1900.

Which sea north of Scandinavia is named after a Dutch navigator who died in 1597 seeking a northeast passage?

The Barents Sea honours Willem Barentsz, who died on the voyage home after his crew spent almost a year stranded on the island of Novaya Zemlya.

Venice stands on a lagoon at the northern end of which sea, a long arm separating Italy from the Balkans?

The Adriatic Sea stretches about 800 km from north-west to south-east and holds more than 1,300 islands, most of them along its eastern coast.

Which sea between Greece and Turkey holds the Cyclades and the Dodecanese island groups?

The Aegean Sea is said to be named after Aegeus, the Athenian king who leapt into it believing his son Theseus had died fighting the Minotaur.

The Isle of Man, a self-governing Crown Dependency, lies in the middle of which sea between two larger islands?

The Irish Sea has two big islands of its own: Anglesey, off North Wales, is the largest, and the Isle of Man comes second.

Which South African headland is the true southern tip of the continent, where two oceans officially meet?

Cape Agulhas means 'Cape of Needles' in Portuguese, a name said to recall the compass needles that pointed to true north there around 1500.

The spot where the equator crosses the prime meridian, nicknamed Null Island, lies in which waters off West Africa?

The Gulf of Guinea holds 0°N 0°E, a patch of open water roughly 600 km from the coast. There is no island there; a weather buoy was moored at the spot instead.

Which bay, fed by the Ganges and Brahmaputra rivers, is the largest bay in the world?

The Bay of Bengal covers about 2.6 million square kilometres, more than twice the area of Hudson Bay. Ancient Greeks and Romans called it the Gulf of the Ganges.

Earth's largest waterfall flows unseen underwater, a drop of about 3.5 km, beneath which passage west of Iceland?

The Denmark Strait cataract forms where frigid, dense Nordic water sinks under warmer water and spills over a drop in the seabed. It is more than three times the height of any waterfall on land.

Which Norwegian explorer made the first complete voyage through the Northwest Passage, aboard the small sloop Gjøa?

Roald Amundsen took from 1903 to 1906 to complete the crossing. He went on to lead the first expedition to reach the South Pole, in 1911.

Which Spanish navigator brought the ship Victoria home to Seville in 1522, completing the first voyage around the world?

Juan Sebastián Elcano had left Spain aboard a different ship, one of five that set out. Only 18 Europeans and 3 Moluccans were still aboard when Victoria reached Seville.

Which Portuguese navigator opened the sea route from Lisbon to India by rounding the Cape of Good Hope in 1497–98?

Vasco da Gama crossed from Malindi in Kenya to Kozhikode in 1498. He was not the first to round the Cape: Bartolomeu Dias had done that in 1488.

Which Nova Scotia-born sailor became the first person to sail alone around the world, aboard the sloop Spray?

Joshua Slocum's book about the voyage became a bestseller in many countries. In 1909 he set out again aboard the same boat and disappeared.

Which Norse explorer is thought to have landed in North America about 500 years before Columbus crossed the Atlantic?

Leif Erikson founded a Norse settlement at a place the sagas call Vinland, usually taken to mean the coast of North America.

Which Ming dynasty admiral commanded seven 'treasure voyages' that took Chinese fleets as far as East Africa?

Zheng He led the seven expeditions between 1405 and 1433 on behalf of the Ming court, trading and collecting tribute along the way.

Which American merchant ship was found adrift and deserted off the Azores in 1872, her crew never seen again?

Mary Celeste was still seaworthy and under partial sail, with her cargo of alcohol intact and the lifeboat missing. An inquiry failed to establish what had happened.

Which 18th-century clockmaker solved the problem of finding longitude at sea by inventing the marine chronometer?

John Harrison was a joiner by trade. He pressed for Parliament's £20,000 longitude reward for decades, and won his last payment at the age of 80 only after appealing to King George III.

Which US Navy officer, nicknamed 'Pathfinder of the Seas', wrote the first comprehensive book on oceanography in 1855?

Matthew Maury studied thousands of ships' logs and turned them into wind and current charts that drastically shortened long sea voyages.

What legal term covers the waters beyond any country's jurisdiction, which make up over two-thirds of the ocean?

The high seas belong to no state, under a doctrine known as the freedom of the seas. Together they cover about half the surface of the planet.

Which unit of water depth, equal to six feet, began as the span of a man's outstretched arms?

Fathom comes from a word the Anglo-Saxons used for a pair of embracing arms. Its size once varied slightly, depending on whether it was tied to the nautical mile or to the yard.

Which country, spread across as many as 17,000 islands between two oceans, is the world's largest archipelagic state?

Indonesia takes in Sumatra, Java, Sulawesi and part of Borneo. It is also the 14th-largest country in the world by area.

Which island nation south-west of India, a double chain of coral atolls, is Earth's lowest-lying country?

The Maldives averages only about 1.5 metres above sea level, which leaves its roughly 1,190 islands especially exposed to any rise in the sea.

Which British-ruled South Atlantic island is considered the most remote inhabited place in the world?

Tristan da Cunha has no airstrip, so the only way to reach or leave it is by ship. It is governed as part of one territory with Saint Helena and Ascension.

Which Pacific archipelago on the equator, about 900 km off South America, inspired Darwin's theory of natural selection?

The Galápagos Islands belong to Ecuador, which protects all of them as a park, together with a reserve covering the surrounding sea.

In Greek myth the Titan Oceanus was the god of what, believed to encircle the entire world?

A great river was thought to circle the world, and Oceanus was its god. He and his sister-wife Tethys were the parents of the river gods, three thousand of them by Hesiod's count.

On World Oceans Day 2021, which map publisher began recognising the Southern Ocean as the world's fifth ocean?

National Geographic had shown only four oceans since it began making maps in 1915. Its cartographers said the swift current circling Antarctica keeps those waters distinct.

The Atlantic Ocean's basin, winding between the Americas and the Old World, is commonly described as having what shape?

S-shaped and second in size among the oceans, the Atlantic covers about 17% of Earth's surface and nearly a quarter of its water.

Which channel between Sicily and mainland Italy is linked to the legend of the sea monsters Scylla and Charybdis?

The Strait of Messina is just 3.1 km wide at its narrowest. A crag at the town of Scilla and a natural whirlpool in its northern part have been linked to the legend.

Which African country's foggy Atlantic shore, once littered with the bones of sea mammals, is called the Skeleton Coast?

Namibia's Skeleton Coast is wrapped in dense sea fog for much of the year, and rocks offshore have wrecked many ships. Portuguese sailors once called it the Gates of Hell.

Which arm of the sea, about 1,700 km long, separates the island of Madagascar from the mainland of Africa?

The Mozambique Channel is 419 km across at its narrowest and reaches depths of more than 3,000 metres. The Comoro Islands mark its northern entrance.

Krakatoa, the volcano that exploded catastrophically in 1883, stands in which waterway between Java and Sumatra?

The Sunda Strait is dotted with islands and links the Java Sea with the open ocean. It is named after the Sunda Kingdom, which ruled western Java for about nine centuries.

The shipwreck-strewn waters off which US state's Outer Banks are nicknamed the Graveyard of the Atlantic?

North Carolina's shifting sands and the shoals off Cape Hatteras have claimed thousands of ships, among them the Civil War ironclad USS Monitor.