This global, interconnected body of salt water, called the World Ocean, is divided by the continents and archipelagos into the following four bodies, from the largest to the smallest: the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, and the Arctic Ocean, and, according to some authorities such as International Hydrographic Organization(IHO), a fifth ocean, the Southern Ocean.
Some geographers and some governments but not the US, recognize the IHO as defining official water body names and boundaries. (The US authority is the United States Board on Geographic Names.) The IHO officially sanctioned the Southern Ocean name only in 2000, but its definition by a line of latitude (with IHO members widely disputing which line of latitude) has left its acceptance as a fifth ocean open to question. The National Geographic Society and some other leading geographers and cartographers continue to use "South Pacific", "South Atlantic", and "Indian" Ocean for the waters around Antarctica. A few Oceanographers recognize only four oceans also, treating the Arctic Ocean (or the Arctic Sea) as a part of the Atlantic Ocean.
Geologically, an ocean is an area of oceanic crust covered by water. Oceanic crust is the thin layer of solidified volcanic basalt that covers the Earth's mantle where there are no continents. From this point of view, there are three "oceans" today: the World Ocean, and the Black and Caspian Seas that were formed by the collision of Cimmeria with Laurasia. The Mediterranean Sea is very nearly its own "ocean", being connected to the World Ocean through the Strait of Gibraltar, and indeed several times over the last few million years movement of the African Continent has closed the strait off entirely, making the Mediterranean a fourth "ocean". (The Black Sea is connected to the Mediterranean through the Bosporus, but this is in effect a natural canal, cut through continental rock some 7000 years ago, rather than a piece of oceanic sea floor like the Strait of Gibraltar.)
The area of the World Ocean is 361 million km², its volume is 1370 million km³, and its average depth is 3790 m. Nearly half of the world's marine waters are over 3000 m deep. 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.
- Main article: Ocean exploration
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 naval 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.
- cetacea such as whales, dolphins and porpoises,
- cephalopods such as the octopus
- crustaceans such as lobsters and shrimp
- marine worms
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.
Continental drift has reconfigured the Earth's oceans, joining and splitting ancient oceans to form the current oceans. Ancient oceans include:
- Panthalassa, the vast world ocean that surrounded the Pangaea supercontinent.
- Tethys Ocean, the ocean between the ancient continents of Gondwana and Laurasia.
- Iapetus Ocean, the southern hemisphere ocean between Baltica and Avalonia.
- Ocean Explorer - An educational and reference resource from NOAA
- Science taps into ocean secrets
- Why is the ocean salty?
- Official IHO boundaries of Oceans and Seas
- The Hydrogen Expedition The first circumnavigation of the globe in a hydrogen fuel cell powered boat
- NOPP - The National Oceanographic Partnership Program