- For other uses, see Ocean (disambiguation).
Ocean (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 is divided by the continents and
- Atlantic Ocean
- Indian Ocean
- Pacific Ocean
- Southern Ocean — in some regions and cultures, including North America and most of Continental Europe, the Southern Ocean is generally not considered an ocean in its own right, and is divided among the Atlantic, Indian and Pacific oceans.
The area of the global ocean is 361 million square kilometres, its volume is 1370 million cubic kilometres, and its average depth 3790 metres. This does not include seas not connected to the oceans, 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.
The boundaries between the oceans are set by the International Hydrographic Organization; e.g., the Southern Ocean extends from the coast of Antarctica to 60 degrees south latitude. Smaller regions of the oceans are known as seas, gulfs, straits, etc.
See sea water for a detailed discussion of ocean water composition, most notably its salinity.
Study of Earth's oceans is called oceanography. 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)  (http://www.rain.org/ocean/ocean-studies-challenger-deep-mariana-trench.html). 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".
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. For example, even though western Europe lies at about the latitude of the northeastern United States or southeastern Canada, it has a more temperate climate than would be expected because of the Gulf Stream, an ocean current that transfers warm, tropical air to western Europe.
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.
- Science taps into ocean secrets (http://news.bbc.co.uk/2/hi/science/nature/4033555.stm)
- Why is the ocean salty? (http://www.palomar.edu/oceanography/salty_ocean.htm)
- Official IHO boundaries of Oceans and Seas (http://ioc.unesco.org/oceanteacher/resourcekit/M3/Formats/Geography/OceansSeas.htm)
- Coreocean (http://www.coreocean.org)
- NOPP - The National Oceanographic Partnership Program (http://www.nopp.org/)