Ranked by their deepest point, the five oceans are the Pacific (about 10,935 m, at Challenger Deep in the Mariana Trench), the Atlantic (8,376 m, Puerto Rico Trench), the Southern Ocean (about 7,430 m, South Sandwich Trench), the Indian Ocean (about 7,190 m, Java Trench) and the Arctic (about 5,550 m, Molloy Deep). Ranked by average depth the order changes slightly: the Pacific is again deepest at around 4,000 m, and the Arctic is by far the shallowest at around 1,200 m.
The Five Oceans by Depth
| Ocean | Deepest point | Where | Average depth | Area |
|---|---|---|---|---|
| Pacific | ~10,935 m | Challenger Deep, Mariana Trench | ~4,000 m | ~165 million km² |
| Atlantic | ~8,376 m | Milwaukee Deep, Puerto Rico Trench | ~3,650 m | ~106 million km² |
| Southern | ~7,434 m | Factorian Deep, South Sandwich Trench | ~3,300 m | ~21 million km² |
| Indian | ~7,192 m | Java (Sunda) Trench | ~3,700 m | ~70 million km² |
| Arctic | ~5,550 m | Molloy Deep, Fram Strait | ~1,200 m | ~15.5 million km² |
The deepest points here are the figures from the Five Deeps Expedition of 2018–19, the first to send a crewed submersible to the bottom of all five oceans. Older sources often give the Indian Ocean a deeper maximum, about 7,450 m, from earlier surveys of the Java Trench, and list it above the Southern Ocean. That disagreement is the only real change in the ranking. Across all five, the ocean averages about 3,700 m deep.
Why the Numbers Keep Changing
Nobody measures the deepest point of the ocean with a rope. Depth is measured by sonar, which times how long a sound pulse takes to reach the bottom and return. The answer depends on the speed of sound through water, which varies with temperature, salinity and pressure, so every measurement of a deep point carries an error of several metres. Challenger Deep has been given as anything from 10,902 m to 10,994 m. A 2021 study combining sonar and pressure readings from submersible dives put it at about 10,935 m, give or take a few metres. The deepest point on Earth is therefore known only to within about the height of a two-storey house.
The Pacific: Challenger Deep and Its Rivals
The Mariana Trench curves for about 2,500 km east of the Mariana Islands, where the old, heavy Pacific Plate dives beneath the Mariana Plate. Its deepest pocket, Challenger Deep, is named after the British survey ship HMS Challenger, whose crew sounded it in 1951. Mount Everest placed at the bottom would still have more than 2 km of water above its summit, and the pressure is over 1,000 times that at the surface. Our guide to how deep the Mariana Trench is goes into the measurements.
For half a century only two people had been there: Jacques Piccard and Don Walsh, in the bathyscaphe Trieste in January 1960. James Cameron made a solo dive in 2012. Since 2019 Victor Vescovo's submersible, the Limiting Factor, has carried dozens of people down. They include Kathryn Sullivan, who in 2020 became the first person to have both walked in space and reached the deepest point of the ocean. The Pacific also holds the next deepest trenches: the Tonga Trench, whose Horizon Deep reaches about 10,800 m, and the Philippine and Kermadec trenches, both more than 10,000 m deep.
The Atlantic: Milwaukee Deep
The Atlantic's deepest point is in the Puerto Rico Trench, north of the island, where the North American Plate slides past and beneath the Caribbean Plate. Milwaukee Deep, at about 8,376 m, takes its name from the cruiser USS Milwaukee, which located it in 1939. The trench is a hazard as well as a record. The plate boundary here can produce large earthquakes, and scientists monitor it for tsunami risk to Puerto Rico and the eastern Caribbean.
Most of the Atlantic is far shallower. Down its centre runs the Mid-Atlantic Ridge, where the ocean floor is splitting apart at about 2.5 cm a year, widening the ocean. It is the best-known section of the global mid-ocean ridge system, which is about 65,000 km long and is the longest mountain chain on Earth, almost all of it underwater.
The Southern Ocean: The Newest Ocean
The Southern Ocean is the water around Antarctica south of 60° S. The International Hydrographic Organization proposed recognising it in 2000, but that draft was never formally ratified. It entered common use gradually, and National Geographic added it to its maps as a fifth ocean in 2021. Its deepest point lies at the southern end of the South Sandwich Trench: Factorian Deep, at about 7,434 m, first reached in 2019. The trench as a whole goes deeper, to over 8,200 m, but that part lies north of 60° S and therefore in the Atlantic.
The Southern Ocean's defining feature is not depth but motion. The Antarctic Circumpolar Current flows eastward around the continent without meeting land. Driven by the strong westerly winds that sailors called the Roaring Forties and Furious Fifties, it is the largest ocean current on Earth, carrying more than a hundred times the flow of all the world's rivers combined, and it links the Atlantic, Indian and Pacific oceans. More is in our guide to the Southern Ocean.
The Indian Ocean: The Java Trench
The Indian Ocean's deepest point is in the Java Trench, also called the Sunda Trench, south of Indonesia, where the Indo-Australian Plate slides beneath the Sunda Plate. The Five Deeps sonar survey put its deepest point at about 7,192 m. The same plate boundary produced the magnitude 9.1 Sumatra–Andaman earthquake of 26 December 2004. The tsunami it triggered killed about 230,000 people in 14 countries, as far away as Somalia. The Indian Ocean is also the warmest of the five, which feeds the monsoon and the cyclones of the Bay of Bengal and Arabian Sea.
The Arctic: Shallowest by Far
The Arctic Ocean is the smallest and shallowest of the five, and one reason is that much of it is continental shelf. The shelves off Siberia are among the widest in the world, and large areas of the ocean are less than 200 m deep. The deep basins lie in the centre, divided by the Lomonosov Ridge. In 2007 a Russian expedition used Mir submersibles to plant a titanium flag on the seabed at the North Pole, about 4,260 m down, a gesture tied to Russia's claim that the ridge is part of its continental shelf.
The Arctic's sea ice is shrinking fast: at its September minimum it now covers roughly 40 per cent less area than when satellite records began in 1979. The deepest point, Molloy Deep, is not near the pole. It lies in the Fram Strait between Greenland and Svalbard, where the Arctic connects to the Atlantic, at about 5,550 m.
Why the Oceans Differ in Depth
The depth of the open ocean depends mainly on the age of the sea floor beneath it. New ocean crust forms at mid-ocean ridges, hot and buoyant, so the water above the ridges is only about 2,500 m deep. As the crust moves away from the ridge it cools, becomes denser and sinks, so older sea floor lies deeper. The oldest sea floor, about 180 million years old, is in the western Pacific near the Mariana Trench, one reason the Pacific is the deepest ocean on average. The Atlantic is younger and still widening, with a large ridge down its middle. The Arctic is shallow mostly because so much of it is shelf.
The very deepest points all lie in trenches, where old, heavy ocean floor bends down into the mantle at a subduction zone. That is why the deepest point in each ocean is a trench, not the middle of an abyssal plain.
Life at the Bottom
The zone below about 6,000 m is called the hadal zone, after Hades, and it exists almost only in trenches. Life there is sparse but real. Amphipods, small crustaceans related to sand hoppers, have been collected from the floor of Challenger Deep, and single-celled organisms called xenophyophores build shells several centimetres across out of sediment. In 2022 scientists filmed a snailfish at 8,336 m in the Izu–Ogasawara Trench south of Japan, the deepest fish ever recorded. Research suggests fish cannot live much deeper than about 8,200 to 8,400 m, because the molecule they use to protect their proteins from pressure would make their bodies too salty below that depth. Every one of these animals lives under hundreds of times the pressure at the surface, in complete darkness, at temperatures of about 1 to 4 °C.
How Much Is Still Unmapped
Everything above rests on a sea floor that is still mostly unmapped in detail. The Seabed 2030 project, launched in 2017 by the Nippon Foundation and GEBCO, estimated that only about a quarter of the ocean floor had been mapped at high resolution by 2024. The rest is known only from satellite measurements of the sea surface, which reveal large features but miss anything smaller than a few kilometres across. The deepest trenches are among the best-studied places in the deep ocean, as our ranking of the deepest ocean trenches shows.