<P> The borders of the oceans are the limits of the Earth's oceanic waters . The definition and number of oceans can vary depending on the adopted criteria . </P> <P> Though generally described as several separate oceans, the world's oceanic waters constitute one global, interconnected body of salt water sometimes referred to as the World Ocean or global ocean . This concept of a continuous body of water with relatively free interchange among its parts is of fundamental importance to oceanography . </P> <P> The major oceanic divisions are defined in part by the continents, various archipelagos, and other criteria . The principal divisions (in descending order of area) are the: Pacific Ocean, Atlantic Ocean, Indian Ocean, Southern (Antarctic) Ocean, and Arctic Ocean . Smaller regions of the oceans are called seas, gulfs, bays, straits, and other names . </P> <P> 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 . Continental crust is thicker but less dense . From this perspective, the Earth has three oceans: the World Ocean, the Caspian Sea, and the Black Sea . The latter two were formed by the collision of Cimmeria with Laurasia . The Mediterranean Sea is at times a discrete ocean, because tectonic plate movement has repeatedly broken its connection to the World Ocean through the Strait of Gibraltar . The Black Sea is connected to the Mediterranean through the Bosporus, but the Bosporus is 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 . </P>

Where does one sea end and another begin
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