If something defines the earth, it is a "blue planet". Water occupies 71% of its surface. The ocean was formed around 4,000 million years ago. Then, when the tectonic plates separated, they divided the five big oceans that we currently know: The Pacific, the Atlantic, the Indian, the glacial Arctic and the glacial Antarctic.
Before developing the navigation and mapping techniques, human beings saw the ocean as distinct units. In each ocean, there are delimited areas, close to the continents or even interiors, which form the regional seas. The peninsulas, islands and archipelagos are geographical accidents that mark the presence of terrestrial areas in the oceanic immensity.
The oceans along with rivers and lakes form the hydrosphere; that is to say, the global set of waters of the planet earth. The oceans contain 96% of liquid water in the Earth. It means that just 4% is fresh water.
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