The Ocean Basins and the Seafloor

Earth's single continuous body of salt water is conventionally divided into five ocean basins, each defined by the continents that bound it and the currents that circulate within it. The Pacific Ocean is by far the largest and deepest, covering roughly a third of the planet's surface and holding an average depth of around 4,000 meters; it stretches from the western coasts of the Americas to East Asia and Australia. The Atlantic Ocean, the second largest, is distinguished by its elongated, S-shaped basin and by the Mid-Atlantic Ridge running down its center, separating the Americas from Europe and Africa. The Indian Ocean, the third largest, is largely enclosed by Africa, Asia, and Australia and is notable for being the warmest of the major oceans. The Arctic Ocean, smallest and shallowest of the five, is centered on the North Pole and remains substantially covered by sea ice for much of the year. In 2021 the U.S. National Geographic Society formally recognized a fifth basin, the Southern Ocean, defined not by continental coastlines but by the Antarctic Circumpolar Current, generally placed at about 60 degrees south latitude, encircling Antarctica. Each basin hosts distinctive current systems, depth profiles, and marine ecosystems, and together they hold roughly 97 percent of all the water on Earth's surface.