Ocean Quiz
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8correct to beat the crowd
This ocean and marine life trivia quiz has 188 free questions with answers and goes under the sea from the tide pools to the trenches: sea creatures from clownfish and octopuses to blue whales and anglerfish, how currents and tides work, the five oceans and their deepest points, coral reefs, and the explorers and submersibles that mapped the deep. About a third of the questions are easy enough for kids and classrooms, the rest climb toward marine biology stumpers, so it works as a family round, a school activity or a pub-quiz sea round. Every answer has been checked against a real source and each question shows its citation.
30 of 188 questions with answers and explanations. Play the quiz
Q 01About what share of Earth's surface is dry land?
About 29%
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.
Q 02Which traditional unit of six feet do sailors use to measure the depth of water?
Fathom
Fathom comes from an Old English word for outstretched arms, because one fathom is roughly the span between a grown man's fingertips.
Q 03Roughly how deep is the ocean on average?
About 3.7 kilometres
About 3.7 kilometres: sunlight fades out within the first 1,000 metres, so most of the seabed lies in permanent darkness.
Q 04What is the average salinity of ocean water?
About 3.5%
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.
Q 05Which body of water experiences the highest tidal range on Earth?
Bay of Fundy
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.
Q 06Divers who breathe compressed air and surface too fast get 'the bends' when which gas bubbles out of their blood?
Nitrogen
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.
Q 07What do we call the extra-high tides that arrive when the Sun and Moon line up with Earth?
Spring tide
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.
Q 08What is the global ocean 'conveyor belt' of density-driven deep currents called?
Thermohaline circulation
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.
Q 09Which warm ocean current flows northward along the eastern coast of the United States?
Gulf Stream
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.
Q 10Males of which whale species are known for singing long, complex songs?
Humpback whale
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.
Q 11Most of the animals filmed in the open water off Monterey, California, share which ability?
Bioluminescence
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.
Q 12What is the primary compound that makes seawater salty?
Sodium chloride
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.
Q 13Which ocean zone is known as the "twilight zone" because only faint light reaches it?
Mesopelagic
Q 21Which of the world's oceanic divisions is the smallest by area?
Arctic Ocean
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.
Q 22The Mid-Atlantic Ridge rises above the sea to form which island country?
Iceland
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.
Q 23What is a tsunami primarily caused by?
Underwater earthquakes
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.
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.
Q 14Which mineral compound, also the stuff of seashells, do corals secrete to build the Great Barrier Reef?
Calcium carbonate
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.
Q 15Which penguin, the largest living species, breeds on the Antarctic ice in the depths of winter?
Emperor penguin
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.
Q 16By mid-2025, about how much of the ocean floor had been mapped in detail by sonar?
About 25%
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.
Q 17Which gas does the ocean absorb from the atmosphere, contributing to ocean acidification?
Carbon dioxide
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.
Q 18Which explorer made the first solo descent to the bottom of the Mariana Trench in 2012?
James Cameron
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.
Q 19Which is the largest animal known to have ever lived on Earth?
Blue whale
Blue whale: the giant feeds almost entirely on krill, shrimp-like animals only a few centimetres long.
Q 20What do female deep-sea anglerfish use their glowing head bulb for?
Attracting prey
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.
Q 24What is the term for the partly enclosed coastal water where a river meets the sea and fresh water mixes with salt?
Estuary
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.
Q 25What is the Ring of Fire?
A belt of earthquakes and volcanoes rimming the Pacific
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.
Q 26What are hydrothermal vents?
Seabed fissures that discharge geothermally heated water
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.
Q 27What type of organisms form the base of most ocean food chains?
Phytoplankton
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.
Q 28What is the Great Pacific Garbage Patch?
A gyre of drifting debris trapped by currents
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.
Q 29What is the Sargasso Sea unique for among the world's seas?
It has no land boundaries
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.
Q 30Which shipboard instrument replaced the weighted lead line for measuring water depth?
Echo sounder
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.