Oceans

One connected ocean

NOAA, the US agency that monitors the oceans and atmosphere, says the ocean covers approximately 70 percent of Earth’s surface; an older USGS geology text puts it at about two thirds.^{[1, 4]} NOAA gives its surface area as about 360 million km² and its average depth as 3,682 m.^[1] That average is an estimate, calculated in 2010 from satellite measurements.^[2]

The ocean stores about 1,338,000,000 km³ of water, about 96.5 percent of all the water on Earth.^[3] Seawater carries about 35,000 parts per million of salt.^[3] More than 90 percent of the ocean is “deep ocean”, meaning deeper than 200 m.^[1]

The seafloor and the mid-ocean ridge

Before the 19th century, most people thought the ocean floor was fairly flat and featureless.^[4] In 1855 a depth chart published by US Navy Lieutenant Matthew Maury showed the first evidence of underwater mountains in the central Atlantic.^[4] Later, echo sounders timed how long a “ping” of sound took to bounce off the bottom and return, and their records showed a far more rugged floor than expected.^[4]

Surveys by many nations then found a great mountain range on the ocean floor that virtually circles the Earth.^[4] NOAA describes this mid-ocean ridge system as nearly 65,000 km long, with more than 90 percent of it underwater; an older USGS text says more than 50,000 km.^{[5, 4]}

The ridges lie where tectonic plates pull apart and new ocean floor is created.^[5] The geologist Harry Hess proposed that new crust spreads away from the ridges like a conveyor belt, and Robert Dietz coined the term “seafloor spreading”.^[4] The plate story itself belongs to Geology.

How much is mapped?

A multibeam sonar sends many sound beams at once in a fan shape, and the seafloor depth is worked out from the time each sound pulse takes to reach the bottom and return.^[6] As of April 2026, 28.7 percent of the global seafloor had been mapped this way, in high resolution; the earlier NOAA figure was about 26 percent in June 2024.^{[1, 2]} Satellites have mapped the entire seafloor, but only as a general picture.^[1]

The international Seabed 2030 project, launched in 2017, aims to “produce the definitive map of the world ocean floor by 2030.”^[7] NOAA also estimates that explorers have seen less than 0.001 percent of the deep seafloor directly.^[1]

Layers and sound

Near the surface, wind and waves keep a layer of warmer water mixed.^[8] Below it lies the thermocline, a transition layer between that warm mixed water and the cooler deep water.^[8] It is semi-permanent in the tropics, variable in temperate regions and shallow to nonexistent near the poles, where the water is cold from top to bottom.^[8]

In the “sound channel”, sound waves are refracted up and down and can travel thousands of miles without losing much energy.^[9]

How the ocean moves

Wind, tides, and differences in temperature and salinity drive ocean currents.^[10] Winds also drive strong currents along the western sides of ocean basins; the most prominent are the Gulf Stream, the Agulhas and the Kuroshio.^[11] The Gulf Stream, the largest by volume, carries more water than all the world’s rivers combined.^[11] NOAA counts five major subtropical gyres, the great loops of circulating current in the open ocean.^{[10, 11]}

The Gulf Stream is part of the Atlantic Meridional Overturning Circulation (AMOC), which carries warm surface water north and cool deep water south.^[11]

Along coasts, winds can push surface water away so that colder, nutrient-rich water rises to replace it, a process called upwelling; the reverse, where surface water piles up and sinks, is downwelling.^[12] Since 1999, a project called ECCO has applied the laws of physics to satellite and float data to build a continuous ocean model spanning decades.^[11]

The ocean in the water cycle

The ocean is estimated to supply about 90 percent of the water that evaporates into the water cycle.^[3]

The ocean and climate: what has been measured

Ocean heat has risen steadily since measurements began in 1955, setting records in 2023.^[13] The ocean’s estimated share of the extra heat from human-caused warming is around 90 percent on NASA’s climate site and more than 90 percent on NOAA’s Climate.gov.^{[13, 14]} Climate.gov reports that over 1993–2024 the full-depth ocean gained heat at about 0.66 to 0.74 watts per square metre of Earth’s surface, the range covering different datasets.^[14] Data also come from more than 3,000 robotic Argo floats, which rise through the water about every 10 days, recording temperature and salinity.^[14]

NOAA estimates that the ocean absorbs about 30 percent of the carbon dioxide (CO₂) released into the atmosphere.^[15] It takes CO₂ up through phytoplankton and by simple dissolving, but not evenly: some regions take it in while others release it.^[16] Dissolved CO₂ sets off reactions that make seawater more acidic and leave fewer carbonate ions, a building block of shells and coral skeletons.^[15] This slow drop in pH is called ocean acidification.^[15]

Single studies show how variable uptake can be: in 2007 Ute Schuster and Andrew Watson reported that North Atlantic carbon uptake halved between 1994 and 2005, and in 2006 a team led by Feely reported that the equatorial Pacific released more CO₂ in 1997–2004 than before.^[16] These are dated findings, and a 2008 NASA article noted that estimating the ocean carbon sink from atmospheric data was uncertain.^[16]

Sea level

Sea level is measured mainly with tide stations, which record local water height along coasts, and with satellites, which measure the average height of the whole ocean.^[17] The satellites time a radar signal bouncing off the sea surface, and tide stations check their accuracy.^[18] In 2014, global sea level was 2.6 inches above its 1993 average, the highest annual average in the satellite record at that time.^[17] The two main causes of rise are warming water expanding and land ice melting, and local rise can differ from the global average.^[17]

Over longer spans, sea level has changed far more: in the last ice age the oceans stood about 122 m lower than today, and about 125,000 years ago they were about 5.5 m higher.^[3]

Oxygen and life

Scientists estimate that roughly half of Earth’s oxygen production comes from the ocean, though marine life consumes roughly the same amount, and the exact share is hard to pin down because it keeps changing.^[19] Scientists estimate the ocean may hold between 700,000 and 1 million species.^[1]

Common misconceptions

“Satellites have mapped the whole seafloor in detail.” They have mapped it all, but only coarsely.^[1]

“Melting sea ice raises sea level.” It does not, although it does affect global temperatures.^[13]

“A gyre is a garbage patch.” The word is sometimes used for areas of concentrated plastic and debris, but in oceanography a gyre is a loop of circulating current.^{[10, 11]}

What this page does not cover

Tides and Ocean Currents have their own pages. Waves, tsunamis and storms belong to Natural Hazards, and the variety of ocean life to Ecology.

Going further

See Geology for plate tectonics and Weather & Climate for the wider climate picture.