The ocean is one connected body of water, even though maps divide it: there is really one connected world ocean, and the boundaries between its named parts have changed over time for historical, cultural, geographical and scientific reasons. It is usually split into five named parts, the Pacific, Atlantic, Indian, Arctic and Southern oceans, and the Southern Ocean is the newest named ocean, the water around Antarctica, as of November 2026, although not every country agrees on its northern boundary. The ocean holds about 97 percent of all the water on Earth, and the Pacific Ocean holds more than half of the free water on Earth and has the oldest ocean rocks, about 200 million years old. About 3.5 percent of the weight of sea water is dissolved salt, which comes mostly from rocks on land, carried to the sea by rain and rivers.
Seen in cross-section, the ocean is a sequence of layers defined by light, temperature and pressure, and sunlight, temperature and pressure all change with depth. Sunlight reaches only about the top 200 meters (656 feet) of the ocean, and the twilight zone is the layer from 200 to 1,000 meters (about 650 to 3,300 feet) down, just beyond the reach of sunlight. Light fades out below about 1,000 meters, temperatures fall toward 4 degrees Celsius, and pressure rises all the way to the seafloor. The edges of these zones are conventions, and sources draw them differently: WHOI puts the abyssal zone at 3,000 to 6,500 meters, while NOAA gives 4,000 to 6,000 meters. Most of the deep ocean has never been seen by people, as of November 2026.
The seafloor is not a flat plain but a landscape of ridges and trenches with a known origin. The mid-ocean ridge is a chain of undersea mountains about 40,000 miles (65,000 kilometers) long that winds around the whole Earth, and mid-ocean ridges form where tectonic plates move apart, as hot rock rises from the mantle, melts, and cools into new seafloor. Trenches form at subduction zones, where an older, denser tectonic plate is pushed under a lighter one and bends the seafloor into a steep, V-shaped depression. Trenches make up the deepest 45 percent of the ocean's depth range, and the very deepest parts are only about 1 percent or less of a trench's area, since most of a trench is its steep slopes and walls. Pressure in the deepest trenches is more than 1,000 times the pressure at the surface, and the water is just above freezing.
Measuring the deepest point has been a story of improving instruments and of honest disagreement. The Challenger expedition of 1872 to 1876 is where modern oceanography began, since it was the first voyage to explore the deep sea successfully on a global scale, and its crew sounded the Mariana Trench at about 8,200 meters (26,850 feet), which gave the Challenger Deep its name. On 23 January 1960 Don Walsh and Jacques Piccard rode the bathyscaphe Trieste down to the Challenger Deep, the first people to reach the deepest known point of the ocean. Later surveys put the deepest point of the ocean at about 11,000 meters, deeper than the first sounding, and published depths differ by source and survey: NOAA gives about 10,935 meters (35,876 feet) and WHOI gives 10,911 meters (35,797 feet), as of November 2026.
Hot water also gushes out of the seafloor, and the vents changed what biologists thought life needed. Hydrothermal vents were found in 1977 near the Galápagos Islands; they are openings on the seafloor where water heated by magma flows out, and life there runs on chemical energy instead of sunlight. Water heated by magma under the seafloor picks up metals such as iron, zinc and copper, and where this fluid meets cold sea water, minerals form black chimneys called black smokers, with the fluid reaching 400 degrees Celsius (750 degrees Fahrenheit) without boiling because of the deep ocean's pressure. Many deep-sea animals make their own light; bioluminescence is when a living thing makes light with a chemical reaction, and an anglerfish has a glowing lure that it uses to bring small fish close enough to eat.
Life near the surface depends on plankton, which are small but immensely important. Phytoplankton are tiny plant-like living things in the ocean that use sunlight to make food and give off oxygen, and many sea animals depend on them for food, while about half of the oxygen made on Earth comes from the ocean, mostly from tiny plankton. A large bloom of phytoplankton can cover hundreds of square kilometers, and satellites can see it from space. Zooplankton rise to the surface at night to feed on phytoplankton and sink to deeper water by day, a daily trip that is the largest migration on Earth, and it can be seen from space. Many models predict that phytoplankton production will fall as the ocean surface warms, but scientists are not sure, and whether total production will change remains uncertain.
Tides, currents and storms all come from the ocean's motion. The Moon is much closer to Earth than the Sun is, so it has the stronger pull on the ocean, and the Sun adds a little less than half as much tide-generating force as the Moon. Tides are very long waves: high tide is the crest of the wave reaching a coast and low tide is the trough, and the difference in height between them is the tidal range. Tides depend on more than the Moon and Sun, since continents, the shape of the Earth and the depth of the ocean in different places change when and how high the tide gets in a given spot. A tsunami in deep water can move up to 800 kilometers (500 miles) per hour, and in shallow water it slows down and can rise to 20 to 30 meters (65 to 100 feet) before it hits land.
Surface and deep currents are linked in one system. Wind-driven surface currents and density-driven deep currents are linked in one global system, the ocean conveyor, which carries heat, salt, nutrients and carbon around the world and helps regulate climate. Water that is cold and very salty is heavier, so it sinks, and this sinking water starts slow, deep currents that move around the whole ocean. The Gulf Stream moves more water than all the world's rivers combined, and the Atlantic Meridional Overturning Circulation, or AMOC, carries warm surface water north and cooler water south at depth. El Niño begins when the trade winds weaken: in normal years the winds push warm surface water west across the Pacific and cold water rises along South America, while in El Niño, warm water flows back east and the weather changes around the world.
The ocean is also the planet's main store of heat and carbon, which is why it features so prominently in climate science. The ocean has absorbed about 90 percent of the extra heat that people's greenhouse gas emissions have trapped on Earth since 1971, as of November 2026, and earlier studies underestimated how much heat the Southern Ocean has taken up since the 1970s: new estimates suggest it absorbed 25 to 58 percent more heat than researchers had thought. The ocean takes in roughly a quarter to a third of the carbon dioxide that people release into the air, and how well the ocean can keep doing this is one of the big unknowns about the greenhouse effect. When carbon dioxide dissolves in sea water, it reacts to make more hydrogen ions, which makes the water more acidic and leaves fewer carbonate ions for animals that build shells. Ocean acidification does not hurt every species the same way: in one lab study some shell-builders such as conchs deteriorated in high carbon dioxide while some others thrived.
Sea level shows how a measurement can have several causes and several answers. Sea level is rising mainly because warm ocean water expands and because ice on land melts and runs into the sea, and as the ocean warms, its water expands, and this thermal expansion raises sea level even without any added ice melt. Melting ice on land is now the main source of added water in sea level rise, and NASA reports that melting land ice from Greenland and glaciers accounts for about two-thirds of sea level rise to date, as of November 2026. Sea level rise is not the same everywhere: local factors such as sinking land, ocean currents, and land still rebounding after the weight of Ice Age glaciers can make a place's sea level rise faster or slower than the global average. It is measured two ways, since tide stations record the water height along coasts relative to the land, and satellites measure the average height of the whole ocean.
Corals and the wider living ocean show both fragility and recovery. Corals are animals that catch food with tiny tentacles and share their bodies with tiny algae, and when stressed by changes in temperature, light or nutrients, corals expel the algae that live in their tissues and turn white; this is called bleaching, and a bleached coral is not dead, but it is under stress. Coral reefs cover very little of the ocean floor, but about a quarter of known sea species live in or near them, and they act as a natural wall for coasts, breaking up waves during storms. Scientists know of about 226,000 ocean species, but they estimate the ocean holds 700,000 to 1 million species, so most are still undescribed, as of November 2026. Overfishing has reduced some fish populations, but many fisheries are recovering where management is strong and based on science, and a marine protected area is any part of the ocean that has legal protection.
Plastic in the sea is a problem with a clear starting point, which is one reason people can do something about it. Much of the trash in the ocean starts on land, and microplastics are bits of plastic less than five millimeters long, about the size of a pencil eraser. They come from two main places: pieces made small on purpose, like microbeads, and bits that break off bigger plastic, and they have been found in oceans around the globe and are eaten by small animals at the base of the food web. Sea animals and birds can mistake plastic for food. Dead zones are ocean areas with too little oxygen for most sea life; they can form when an algal bloom dies and the microbes that break it down use up the oxygen faster than it can be replaced. Seagrasses are true flowering plants with roots, not seaweed, and seagrass meadows store an estimated 18 percent of ocean carbon even though they cover just 0.1 percent of the ocean floor.
Words to know
- bioluminescence: living light
- subduction: plate sliding under
- acidification: becoming more acidic
- plankton: tiny drifting life
