Oceans

An ocean is a large body of saline water that covers about 71 percent of the Earth's surface. There are five recognized oceans: the Pacific, Atlantic, Indian, Southern (Antarctic), and Arctic. The Pacific is the largest, followed by the Atlantic and Indian. Together they hold most of the Earth's seawater and play a central role in climate, weather, and the cycling of nutrients. Ocean depths range from shallow shelves to deep trenches, with the deepest point in the Challenger Deep of the western Pacific.

Water in the oceans typically has about 35 parts per thousand salinity, though salinity varies regionally. Temperature and density stratify the water column, creating distinct surface and deep layers. Ocean circulation includes wind-driven surface currents and the global thermohaline circulation, a slow, large-scale movement that transfers heat and carbon between basins and helps regulate climate.

Ocean ecosystems are highly diverse, ranging from microscopic phytoplankton to whales, corals, and squid. Phytoplankton produce a substantial portion of the planet's oxygen. Many organisms depend on upwelling regions, coral reefs, and seafloor habitats such as hydrothermal vent communities.

Human activity affects oceans through overfishing, pollution, plastic debris, and climate change, which causes warming and acidification. The oceans also store heat and carbon, dampening and amplifying climate signals. International law and cooperation, along with marine protected areas and sustainable practices, aim to manage resources, reduce harm, and monitor the health of marine ecosystems.