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This ocean and marine life trivia quiz has 142 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 142 questions with answers and explanations. Play the quiz
Q 01What percentage of Earth's surface is covered by oceans?
About 71%
Oceans cover approximately 71% of Earth's surface, making our planet predominantly a water world when viewed from space.
Q 02Which of the world's oceanic divisions is the largest by area?
Pacific Ocean
The Pacific Ocean covers about 165 million square kilometres, more than all the planet's land area combined.
Q 03What is the deepest known spot on the seabed called?
Challenger Deep
It reaches about 10,935 metres below sea level, deeper than Everest is tall, and is named for the HMS Challenger expedition that first sounded it in 1875.
Q 04What is the average depth of the world's oceans?
About 3,688 meters
The average depth of the world's oceans is approximately 3,688 meters (12,100 feet). This means if you could flatten Earth's surface, the entire planet would be covered in water about 2.5 km deep.
Q 05What is the average salinity of ocean water?
About 3.5%
Every kilogram of seawater carries roughly 35 grams of dissolved salts, enough that a swimmer floats noticeably higher than in a lake.
Q 06Which 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 07What causes ocean tides?
Gravitational pull of the Moon and Sun
The gravitational pull of the Moon and Sun raises bulges of water on Earth; the Moon's tidal effect is about twice the Sun's, so high tides track the Moon's passage.
Q 08What do we call the extra-high tides that arrive when the Sun and Moon line up with Earth?
Spring tide
Spring tides arrive every two weeks, at new and full moon, when the Moon's and Sun's pulls align; the Moon supplies about 69% of the tide-raising force and the Sun 31%.
Q 09The deepest known point of the seabed lies in the ocean territory of which country?
Federated States of Micronesia
The trench itself is named for the Mariana Islands, but its deepest southern end sits in Micronesian waters near Guam.
Q 10What is the global ocean 'conveyor belt' of density-driven deep currents called?
Thermohaline circulation
Thermohaline circulation takes its name from the Greek for heat and salt; a parcel of water takes roughly a thousand years to complete one lap of the belt.
Q 11Which warm ocean current flows northward along the eastern coast of the United States?
Gulf Stream
The Gulf Stream is a powerful, warm ocean current that originates in the Gulf of Mexico, flows through the Straits of Florida, and travels up the U.S. East Coast before veering toward northwestern Europe.
Q 12What is the uppermost zone of the ocean called, where sunlight penetrates?
Epipelagic
The epipelagic zone (also called the sunlight zone) extends from the surface to about 200 meters deep. It receives enough sunlight for photosynthesis and contains the vast majority of ocean life.
Q 13Roughly what share of animals seen in Monterey Bay's open water can produce their own light?
About 75%
Q 21What is the largest animal to have ever lived on Earth?
Blue whale
The blue whale reaches about 30 metres and up to 200 tonnes, heavier than any dinosaur yet discovered.
Q 22What do female anglerfish use their bioluminescent lure for?
Attracting prey
Female anglerfish dangle a glowing, bioluminescent lure (called an esca) from a modified dorsal fin spine on their forehead. The light attracts curious prey in the pitch-black deep sea, drawing them close enough to be snapped up.
Q 23Which of the world's oceanic divisions is the smallest by area?
Arctic Ocean
It is also the shallowest of the five and spends much of the year under sea ice.
About 75%: a 2017 MBARI survey of Monterey Bay's water column found 76% of the animals observed were bioluminescent, using light to lure prey, confuse predators and signal mates.
Q 14What is the primary compound that makes seawater salty?
Sodium chloride
Sodium and chloride ions dominate because they linger in the ocean for tens of millions of years, while calcium is pulled out quickly to build shells and reefs.
Q 15Which ocean zone is known as the "twilight zone" because only faint light reaches it?
Mesopelagic
The mesopelagic zone spans roughly 200 to 1,000 metres: too little light for photosynthesis, but enough for animals to see silhouettes above them.
Q 16What is the Great Barrier Reef primarily composed of?
Coral polyp skeletons
Billions of tiny polyps secrete calcium carbonate that piles up over millennia into a system of more than 2,900 reefs stretching 2,300 km.
Q 17Off the coast of which country is the Great Barrier Reef located?
Australia
The Great Barrier Reef stretches for over 2,300 kilometers along the northeast coast of Australia in the Coral Sea. It is the world's largest coral reef system and is visible from space.
Q 18As of 2024, roughly what share of the ocean floor had been mapped at better than satellite resolution?
About 25%
About 25%: the Seabed 2030 project reported roughly a quarter of the seafloor mapped by ship-based sonar by 2024, leaving the rest known only from coarse satellite-derived gravity data.
Q 19Which gas does the ocean absorb from the atmosphere, contributing to ocean acidification?
Carbon dioxide
The ocean absorbs roughly a quarter of human-produced carbon dioxide (CO2) from the atmosphere. When CO2 dissolves in seawater, it forms carbonic acid, lowering the ocean's pH — a process called ocean acidification.
Q 20Which explorer made the first solo descent to the bottom of the Mariana Trench in 2012?
James Cameron
James Cameron piloted Deepsea Challenger to the Challenger Deep on 26 March 2012, spending roughly 2.5 hours on the bottom, far longer than Trieste's 20 minutes in 1960.
Q 24Which underwater mountain range separates the seafloor of the Americas from that of Europe and Africa?
Mid-Atlantic Ridge
The Mid-Atlantic Ridge marks where the plates pull apart; Iceland is one of the few places it rises above sea level.
Q 25What 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.
Q 26What is the term for the partly enclosed coastal water where a river meets the sea and fresh water mixes with salt?
Estuary
Estuary, and the brackish mix makes it one of the most productive habitats on the planet, a nursery for fish and birds. A delta is the land a river builds; an estuary is the valley the sea has drowned.
Q 27What is the Ring of Fire?
A belt of earthquakes and volcanoes rimming the Pacific
The belt holds around two-thirds of the world's active or dormant volcanoes and most of its largest earthquakes.
Q 28What 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 29What 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 30What is the Great Pacific Garbage Patch?
A gyre of drifting debris trapped by currents
Most of it is fingernail-sized or smaller plastic suspended in the water column, which is why it is invisible from satellites.