In the first hundreds of millions of years after the Earth's formation, due to the thin crust and the constant bombardment of small celestial bodies on the Earth's surface, molten magma in the mantle was easy to rise and erupt. the earth at that time was everywhere a sea of fire. With the magma, a large amount of water vapor and carbon dioxide were ejected. These gases rose into the air and enveloped the earth. Water vapor forms clouds, producing precipitation. After a long period of rain, water continued to accumulate in the lower areas of the original crust, forming the most primitive ocean. There wasn't much seawater in the original ocean, about 1/10 of today's seawater. In addition, seawater in the early ocean was only slightly salty, and the salt content gradually increased afterward. After the gradual increase in the volume of water and thehe salt content, as well as the vicissitudes of geological history, the original ocean gradually formed the current ocean. This is the first representative statement.
Due to the high temperature inside the Earth, it is in a very unstable state. The atmosphere is composed of carbon dioxide and water vapor, but almost no oxygen. When it cools, the water vapor condenses into rain, falls and forms a large pool in a deep depression. It slowly accumulates and becomes the sea.