A sea is a huge amount of salt water which is surrounded in whole or some part by land. In general “the sea” is the connected system of  salty, oceanic waters on our planet — considered as one global ocean or as a few main oceanic divisions. Earth’s climate depends on seas. Also seas play very important roles in the water cycle, carbon cycle, and nitrogen cycle.

In spite of that sea has been traveled and explored by people since prehistory time, the modern science of the sea (oceanography) appeared onle about 1870s. It’s known as the British Challenger expedition. The sea is conventionally divided into up to five large oceanic sections — including the International Hydrographic Organization’s four named oceans (the Atlantic, Pacific, Indian, and Arctic) and the Southern Ocean; smaller such as the Mediterranean, are known as seas.

Deep-sea currents are product of differences in salinity and temperature. Surface currents are generated by the friction of waves which formed by the wind and by tides and the different changing in local sea level. In their return sea levels produced by the gravity between the Moon and Sun.

Former changes in sea levels have left continental shelves, shallow areas in the sea close to land. Human can support their life by fishing. There are a lot of fish, shellfish, mammals, and seaweed, but in the most deepest seas in the
deep waer there is life even under enormous depths and pressures, nutrients streaming from hydrothermal vents support their own unique ecosystem.

The sea is an crucial aspect of human travel, trade, mineral extraction, and power generation. The sea has an another its side. It can be very dangerous for people. A lot of cities and countries are situated near by seas and very  expose earthquakes and volcanoes which are causes of ; powerful tsunami waves; and hurricanes, typhoons, and cyclones.