The sea, also known as the "sea", refers to the large area of salt water connected to the "ocean", that is, the edge of the ocean. Usually large inland salt lakes, large saltwater lakes that are not connected to the ocean, such as the Caspian Sea and the Sea of Galilee, are not “sea”. The sea is divided into the marginal sea, the inland sea and the inland sea (the vast freshwater system, such as the Great Lakes). The sea area is the first bridge for mankind to lead to the ocean. The role of the seas in humans is not only to provide a rich variety of available resources, but also to play an important role in regulating the water balance of the entire planet. The sea is on the edge of the ocean and is a subsidiary of the ocean. The sea area accounts for about 11% of the ocean. The depth of the sea is relatively shallow, with an average depth ranging from a few meters to 3,000 meters. As the sea is close to the mainland, the temperature, salinity, color and transparency of the water are affected by land, and the sea water is frozen in winter. The salinity will be lighter and the transparency will be poor. Compared with the ocean, the sea does not have its own independent tides and currents.