A
sea is a large
body of
salt water that is surrounded in whole or in part by
land. More broadly, "
the sea" is the interconnected system of
Earth's
salty,
oceanic waters—considered as one
global ocean or as several principal oceanic divisions. The sea moderates
Earth's climate and has important roles in the water cycle, carbon cycle, and nitrogen cycle. Although the sea has been travelled and explored since
prehistory, the modern scientific study of the sea—oceanography—dates broadly to the
British Challenger expedition of the 1870s. The sea is conventionally divided into up to five large oceanic sections—including the
IHO's four named oceans (the
Atlantic,
Pacific,
Indian, and
Arctic) and the
Southern Ocean; smaller, second-order sections, such as the
Mediterranean, are known as
seas.
