The continuous expanse of salt water that covers about 70 per cent of the Earth's surface and surrounds the contintental land masses. The ocean formed very early in the history of the Earth, probably as soon as the planet had cooled enough for water to become liquid, about 4 thousand million years ago. The salt came from chemicals released by crustal rocks or from deeper processes, such as those associated with volcanoes. The formation of the continents resulted in the five main oceans known today: Atlantic, Indian, Pacific, Arctic, and Southern (Antarctic). In 2004 the UK's Royal Society launched an investigation into the rising acidity of the world's oceans due to pollution from the greenhouse gas carbon dioxide.
Oceans (from Ὠκεανός Okeanos in Greek, the ancient Greeks noticing the strong current that flowed off Gibraltar and assuming it was a great river) cover almost three quarters (71%) of the surface of the Earth, and nearly half of the world's marine waters are over 3,000 meters (9,800 ft) deep.
This global, interconnected body of salt water, called the World Ocean, is generally divided by the continents and archipelagos into the following bodies, from the largest to the smallest: the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Southern Ocean, and the Arctic Ocean.
Smaller regions of the oceans are called seas, gulfs, straits and other names.
Geologically, an ocean is an area of oceanic crust covered by water. 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 7,000 years ago, rather than a piece of oceanic sea floor like the Strait of Gibraltar.)
Physical properties
The area of the World Ocean is 361 million square kilometers (139 million sq mi), its volume is over 1,340 million cubic kilometers (319 million cu mi), and its average depth is 3,711 meters (12,175 ft). The vast abyssal plains of the deep ocean cover about 40% of the Earth's surface. This does not include seas not connected to the World Ocean, such as the Caspian Sea.
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 Marianas Trench located in the Pacific Ocean near the Northern Mariana Islands.
Much of the bottom of the world's oceans are unexplored and unmapped.
Regions
Oceans are divided into numerous regions depending on the physical and biological conditions of these areas. The pelagic zone includes all open ocean regions, and can be subdivided into further regions categorised by depth and light abundance.
Along with pelagic aphotic zones there are also benthic aphotic zones, these correspond to the three deepest zones. The pelagic zone can also be split into two subregions, the neritic zone and the oceanic zone.
Climate
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.
Ecology
The oceans are home to the majority of plant and animal life on Earth. These lifeforms include:
Radiata fish cetacea such as whales, dolphins and porpoises, cephalopods such as the octopus crustaceans such as lobsters and shrimp marine worms plankton krillEconomy
The oceans are essential to transportation: most of the world's goods are moved by ship between the world's seaports.
Ancient oceans
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.Extraterrestrial oceans
Earth is the only known planet with liquid water on its surface and is certainly the only one in our own solar system.
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;
Liquid hydrocarbons were thought to be present on the surface of Titan, though it may be more accurate to describe them as "lakes" rather than an "ocean". Titan is also thought likely to have a subterranean water ocean under the mix of ice and hydrocarbons that forms its outer crust.
Mythology
The original concept of "ocean" goes back to notions of Mesopotamian and Indo-European mythology, imagining the world to be encircled by a great river, Okeanos, "Ωκεανός" in Greek, Samudra in Hindu mythology (compare also Jörmungandr, the sea serpent living in that outer ocean in Norse mythology). The world was imagined to be enclosed by a celestial ocean above the heavens, and an ocean of the underworld below (compare Rasā, Varuna). This is evidenced for example in the account of Noah's flood in Genesis 7:11, where
all the fountains of the great deep [were] broken up, and the windows of heaven were opened (KJV),inundating the world with the waters of the celestial ocean (see also deluge (mythology)).
One common misconception today is that the oceans are blue primarily because the sky is blue.
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