An ocean (also known as a sea or world ocean) is a body of salt water that covers about 70.8% of the Earth's surface and contains 97% of the water on Earth. An ocean can also refer to any large body of water into which the world ocean is conventionally divided. Distinct names are used to identify five different areas of the ocean:
Pacific (largest), Atlantic, Indian, South (Antarctic) and Arctic (smallest). Sea water covers about 361,000,000 square kilometers (139,000,000 sq mi) of the planet. The ocean is a major component of the Earth's hydrosphere and is therefore an integral part of life on Earth. Acting as a giant heat reservoir, the ocean influences climate and weather patterns, the carbon cycle, and the water cycle.

World map model five oceans with approximate boundaries
Oceanographers divide the ocean into different vertical and horizontal regions based on physical and biological conditions. The flotation zone includes the water column from the surface to the ocean floor through the open ocean. The water column is further classified into other areas based on the depth and amount of light available. The photic zone includes water from the surface down to a depth of 1% of surface light (about 200 m offshore), where photosynthesis can occur. This makes the Photic Zone the richest in biodiversity. The photosynthesis of microscopic plants and algae (free-floating phytoplankton) produces organic matter using light, water, carbon dioxide, and nutrients. Photosynthesis in the oceans produces 50% of the oxygen in the Earth's atmosphere. This upper, sunlight-rich area is the food source that sustains most ocean ecosystems. Light penetrates only a few hundred meters deep; the rest of the ocean below was cold and dark. The continental shelf where the ocean approaches land is shallower, with depths of several hundred meters or less. Human activity has a greater impact on the continental shelf. The temperature of the oceans depends on the amount of solar radiation reaching the ocean surface. In the tropics, surface temperatures can reach more than 30°C (86°F). Near the poles where sea ice forms, the equilibrium temperature is about −2 °C (28 °F). The temperature of deep sea water is between -2°C (28°F) and 5°C (41°F) in all regions of the ocean. Water is constantly circulating in the oceans, creating ocean currents. These directional movements of seawater are caused by forces acting on the water, including temperature differences.

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