noun - 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 traveled 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 International Hydrographic Organization's four named oceans (Atlantic Ocean, Pacific Ocean, Indian Ocean, and Arctic Ocean) and the Southern Ocean; smaller, second-order sections, such as the Mediterranean, are known as seas.