Oceans form a connected body of salt water covering much of the Earth’s surface. Names given to individual oceans help describe different regions, but the water does not stop at those boundaries. Connections between basins allow heat, dissolved substances, and living organisms to move over great distances. The ocean is also closely linked to the atmosphere and to the land through the continuing movement of water.

Conditions vary greatly between the surface and deeper water. Sunlight reaches the upper layers most strongly and becomes weaker with depth. Surface waters respond to weather and can be moved by wind. Far below, the environment is darker, colder in many regions, and subject to greater pressure. The depth and shape of the seafloor introduce further differences, including broad plains, underwater ridges, and deep trenches.

Ocean movement takes several forms. Waves transfer energy across the surface, while currents transport water from place to place. Tides produce a regular rise and fall influenced mainly by the gravitational effects of the Moon and the Sun. These processes overlap, so the water observed along a coast reflects more than one cause. The shape of the coastline and the depth near shore can strongly affect what a local observer sees.

Marine life occupies environments ranging from shallow coastal waters to the deep sea. Some organisms drift with the water, while others swim actively or remain attached to the bottom. Tiny photosynthetic organisms near the surface contribute to food webs that support larger animals. Connections between these organisms are complex, and changes in one part of a marine environment can influence other parts in ways that are not immediately visible.

Studying an ocean requires observations across both distance and time. Measurements from ships, coastal stations, floating instruments, and satellites describe different aspects of the same system. A calm day at one beach cannot represent conditions across an entire basin. Combining local detail with a wider view helps explain why oceans are important to weather, living communities, transport, and the changing conditions along the world’s coasts.