Ocean

The ocean is the vast interconnected body of salt water that covers about 71 percent of Earth's surface and contains the majority of its liquid water. It is conventionally divided into five major basins: the Pacific, Atlantic, Indian, Southern, and Arctic Oceans. While some classifications treat the Arctic differently, most scientific usage recognizes these five as distinct oceans that are connected at the margins.

The mean depth of the ocean is about 3,700 meters, and its deepest point is the Challenger Deep in the western Pacific, at about 10,900 meters. Average salinity is about 35 practical salinity units, though it varies with location and depth. Temperature ranges from well above freezing near the surface in tropical regions to below freezing at the depths in polar regions.

Ocean structure includes distinct zones such as the photic (sunlit) zone and the deeper aphotic zones. Ocean currents, driven by wind, density differences, and Earth's rotation, transport heat and nutrients; major features include the Gulf Stream, thermohaline circulation, tides, and waves. Upwelling brings nutrient-rich waters to the surface, supporting fisheries.

Life in the ocean is highly diverse, with ecosystems including coral reefs, mangroves, kelp forests, sea grass beds, and open-water (pelagic) communities. The ocean is also a major climate regulator and carbon reservoir, absorbing large amounts of atmospheric CO2 and heat.

Humans depend on the ocean for food, transport, energy, and recreation. It faces threats from overfishing, pollution, habitat destruction, and climate change, including acidification and warming. International law, including the United Nations Convention on the Law of the Sea, governs maritime rights and responsibilities, while marine protected areas and sustainable practices seek to conserve ocean health.