The ocean (also called the World Ocean or global ocean) is the connected body of salt water that covers about 71 percent of Earth's surface, roughly 361 million square kilometres. It holds about 97 percent of the planet's water and has a volume of around 1.3 to 1.4 billion cubic kilometres. The ocean stores and moves heat, exchanges gases with the atmosphere and drives the water cycle, so it largely shapes Earth's climate. Most of the space on Earth where organisms can live is in the ocean. People have used it for food, travel and trade for thousands of years.
Divisions and dimensions
The water is all connected, but by convention it is split into five main basins. These are the Pacific, Atlantic, Indian, Southern (Antarctic) and Arctic oceans. The Pacific is by far the largest and holds about half of all ocean water. The Arctic is the smallest. Smaller areas partly enclosed by land, such as marginal seas, gulfs and bays, sit along the edges of the basins and between the continents.
A 2010 estimate based on satellite data gives an average depth of about 3,682 metres. The deepest known point is Challenger Deep in the Mariana Trench in the western Pacific, at about 10,935 metres. Most of the seafloor has been mapped only roughly from satellites. As of 2026, less than a third of it had been surveyed with high-resolution sonar, so a great deal of the deep ocean is still unexplored.
Origin and seafloor
Geologists generally think the ocean formed early in Earth's history. Water released from the planet's interior by volcanoes, and possibly water delivered by comets and asteroids, condensed as the young planet cooled. The shape of the ocean basins is set by plate tectonics. New oceanic crust forms at mid-ocean ridges and sinks back into the mantle at deep trenches. Because of this cycle, the oceanic crust is young in geological terms compared with the continents. The seafloor has many features: continental shelves and slopes, wide abyssal plains, seamounts, and hydrothermal vents, where heated water rich in minerals pours out of the crust.
Seawater properties
Seawater has an average salinity of about 35 grams of dissolved salts per kilogram. The salts are mostly sodium and chloride ions, which come from the weathering of rocks carried to the sea by rivers and from volcanic sources. Near the poles, surface water can drop to about −2 °C before it freezes. In the tropics it can be warmer than 30 °C. Because most of the ocean's water is deep, its average temperature is only about 4 °C. Temperature and salinity together control how dense seawater is, so the ocean forms layers. Sunlight reaches only the top 200 metres or so, the sunlit or epipelagic zone. Below that the water gets steadily darker and colder.
Circulation and tides
Winds blowing over the surface drive large circulating current systems called gyres. These include fast boundary currents such as the Gulf Stream, which carry warm water toward the poles. Deeper water moves through the thermohaline circulation, which is driven by differences in density. Cold, salty water sinks at high latitudes and spreads slowly through the deep basins, linking the oceans into a global "conveyor" that takes centuries to complete. In addition, the ocean rises and falls in tides caused by the gravitational pull of the Moon and the Sun.
Role in climate
Water can absorb a great deal of heat, so the ocean evens out temperature differences between seasons and regions. It also moves large amounts of energy from the equator toward the poles. The ocean is a major part of the carbon cycle. CO₂ dissolves into seawater, and marine organisms take carbon down into deep water and sediments.
Under human-caused climate change, the ocean has taken up about 91 percent of the extra heat trapped by greenhouse gas emissions. It has also absorbed roughly a quarter of the carbon dioxide released by burning fossil fuels and by changes in land use. This has slowed warming in the atmosphere, but it has also caused ocean acidification. Average surface pH fell from about 8.11 in 1985 to 8.04 in 2024. That makes it harder for corals, shellfish and some plankton to build calcium carbonate shells and skeletons. Warmer water also holds less oxygen and expands, which adds to sea level rise along with melting land ice.
Marine life
Life very probably began in the sea, and the ocean is still home to a large share of Earth's biodiversity. Microscopic plankton, including photosynthetic algae and cyanobacteria, form the base of most marine food webs. They produce roughly half of the planet's primary production. Marine ecosystems include coral reefs, kelp forests, the open ocean and the deep sea. At hydrothermal vents, communities of microorganisms live by chemosynthesis instead of using sunlight. Marine bacteria and archaea are extremely numerous and play a central part in cycling nutrients through the ocean.
Human use and exploration
Seafaring opened long-distance trade routes such as the Maritime Silk Road and made the European voyages of exploration possible. These voyages greatly changed how the world was understood and drawn on maps. The steamship and, later, container shipping made ocean transport the main way goods move around the world. The ocean also provides fisheries, offshore oil and gas, and growing sources of renewable energy such as offshore wind.
Scientific oceanography is usually dated to the British HMS Challenger expedition of 1872–1876. Its crew first sounded the depth of the Mariana Trench in 1875, and Challenger Deep is named after the ship. Modern research uses research vessels, satellites, autonomous floats and crewed and robotic submersibles.
Governance
Use of the ocean is governed mainly by the 1982 United Nations Convention on the Law of the Sea (UNCLOS). It sets out territorial seas, exclusive economic zones and rules for the high seas and the deep seabed. The high seas lie beyond any one country's jurisdiction and make up about two-thirds of the ocean. A further agreement under UNCLOS, the Biodiversity Beyond National Jurisdiction (BBNJ) Agreement, often called the High Seas Treaty, was adopted by the United Nations in June 2023. It entered into force on 17 January 2026. It sets up processes for creating marine protected areas, requires environmental impact assessments before harmful activities are approved, and provides for sharing the benefits of marine genetic resources. Major pressures on the ocean include overfishing, pollution such as plastics and nutrient runoff, habitat destruction and climate change. In past mass extinctions, marine life was among the hardest hit.
References
- How deep is the ocean?oceanservice.noaa.gov
- How deep is the ocean? - NOAA Ocean Explorationoceanexplorer.noaa.gov
- How much of the ocean has been explored? - NOAA Ocean Explorationoceanexplorer.noaa.gov
- Oceanen.wikipedia.org
- As Temperatures Climb, the Ocean Absorbs the Impactwri.org
- Ocean acidificationeea.europa.eu
- The Biodiversity Beyond National Jurisdiction (BBNJ) Agreement (or High Seas Treaty)congress.gov
- BBNJ: High Seas Treaty Enters into Force - OceanCareoceancare.org