Ocean (from Okeanos, a Greek god of sea and water; Greek ωκεανός) covers almost three quarters (71%) of the surface of the Earth.
This global, interconnected body of salt water, called the World Ocean, is divided by the continents and archipelagos into the following five bodies, from the largest to the smallest: the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Southern Ocean, and the Arctic Ocean. Official boundaries are defined by the International Hydrographic Organization; the Southern Ocean, though long recognized in maritime tradition, was officially sanctioned only in 2000, and is unique in being defined only by a line of latitude with no landmass boundaries.
Oceanographers, however, may recoginize only four oceans, treating the Arctic Ocean (or the Arctic Sea) as a part of the Atlantic Ocean.
Smaller regions of the oceans are called seas, gulfs, straits and other names.
The area of the World Ocean is 361 million km≤, its volume is 1370 million km≥, and its average depth is 3790 m. This does not include seas not connected to the World Ocean, such as the Caspian Sea.
The total mass of the hydrosphere is about 1.4 × 1021 kg, ca. 0.023 % of the Earth's total mass.
See sea water for a detailed discussion of ocean water composition, most notably its salinity.
Travel on the surface of the ocean through the use of boats dates back to prehistoric times, but only in modern times has extensive underwater travel become possible.
The deepest point in the ocean is the Mariana Trench located in the Pacific Ocean near the Northern Mariana Islands. It has a maximum depth of 10,923 m (35,838 ft) . It was fully surveyed in 1951 by the British navy vessel, "Challenger II" which gave its name to the deepest part of the trench, the "Challenger Deep".
Much of the bottom of the world's oceans is unexplored and unmapped. A global image of many underwater features larger than 10 km was created in 1995 based on gravitational distortions of the nearby sea surface.
One of the most dramatic forms of weather occurs over the oceans: tropical cyclones (also called "typhoons" and "hurricanes" depending upon where the system forms). Ocean currents greatly affect Earth's climate by transferring warm or cold air and precipitation to coastal regions, where they may be carried inland by winds. The Antarctic Circumpolar Current encircles that continent, influencing the area's climate and connecting currents in several oceans.
The oceans are home to many forms of life, such as:
The oceans are essential to transportation: a huge portion of the world's goods are moved by ship between the world's seaports. Important ship canals include the Saint Lawrence Seaway, Panama Canal, and Suez Canal.
Earth is the only planet known with liquid water on its surface, and is certainly the only such in our own solar system. However, liquid water is thought to be present under the surface of several natural satellites, particularly the Galilean moons of Europa, and, with less certainty, its fellows Callisto and Ganymede. Other icy moons may have once had internal oceans that have now frozen, such as Triton. The planets Uranus and Neptune may also possess large oceans of liquid water under their thick atmospheres, though their internal structure is not well understood at this time.
There is currently much debate over whether Mars once had an ocean of water in its northern hemisphere, and over what happened to it if it did; recent findings by the Mars Exploration Rover mission indicate that it had some long-term standing water in at least one location, but its extent is not known.
Liquid hydrocarbons are thought to be present on the surface of Titan, though it may be more accurate to describe them as "lakes" rather than an "ocean". The distribution of these liquid regions will hopefully be better known after the full analysis of data from the Huygens probe of the Cassini-Huygens space mission, which dropped onto Titan's surface in January 2005. Titan is also thought likely to have a subterranean water ocean under the mix of ice and hydrocarbons that forms its outer crust.