The ocean is a large area of salt water, completely or partially surrounded by land. In a broader sense, "ocean" is the interconnected system of salty ocean waters on the earth, which is regarded as a global ocean or several major marine divisions. The ocean alleviates the earth's climate and plays an important role in the water cycle, carbon cycle and nitrogen cycle. Although marine exploration has been carried out since prehistory, modern marine scientific research (Oceanography) can be traced back to the British Challenger expedition in the 1870s. Traditionally, the sea surface can be divided into up to five ocean regions, including the four oceans (Atlantic, Pacific, Indian and Arctic) and the South Ocean of the International Hydrographic Organization. Smaller second-order parts, such as the Mediterranean, are called oceans.

Due to the current state of continental drift, the distribution between land and sea in the northern hemisphere is now fairly average (the ratio is about 2:3), while the vast majority in the south is ocean (1:4.7). Although in more inland waters, near large estuaries or deeper depths, salinity varies within a narrow range of 3.5%, it is usually within a narrow range in the high seas. About 85% of solids in the high seas are sodium chloride. Deep ocean currents are caused by differences in salinity and temperature. The surface current is formed by the friction of waves generated by wind and tide, and the change of local sea level caused by the gravity of the moon and the sun. All of these directions depend on the mass of land on the surface and seafloor and the earth's rotation (Coriolis effect).

Before, the change of sea level made the continental shelf leave the ocean and close to the shallow sea area of the ocean. These nutrient rich water sources are full of life and provide a large amount of food for human beings - mainly fish, as well as shellfish, mammals and seaweed - which are harvested in the wild and on farms. The most diverse areas are surrounded by huge tropical reefs. Whaling was once common in the deep sea, but the decline in the number of whales prompted international conservation efforts and eventually suspended most commercial hunting activities. Oceanography has determined that not all life is limited to sunlight exposed surface water: even at great depths and pressures, nutrients from hydrothermal vents can support their unique ecosystems. Life there may have begun, and aquatic microbial mats are often thought of as oxygenation of the earth's atmosphere. Animals and plants first evolved in the sea.

Ocean is an important aspect of human trade, travel, mining and power generation. It also makes war critical and exposes major cities to earthquakes and volcanoes from nearby faults; strong tsunami waves; and hurricanes, typhoons and cyclones from the tropics. This importance and duality have influenced human culture, from the early God of the sea to Homer's epic, to the transformation of the Columbia exchange, from the sea funeral to the Hai sentence of Basho, to the ultra realistic marine art, and from the inspiration of inspiring music, from Scotland to Rimsky Korsakov's "the sea and the ships of Sinbad's" Ship), then to A-Mei's listening at the sea. It is a place for leisure activities, including swimming, diving, surfing and sailing. However, the growth of population, industrialization and intensive farming all lead to the marine pollution. Carbon dioxide in the atmosphere is being absorbed more and more, reducing its pH in a process called ocean acidification. The shared nature of the oceans makes overfishing an increasingly serious problem.