There was no water on its surfacefor a long time. Its inside was hot that it was a liquid, as much of its still is today. Its outside, too, was probably so frightfully hot that a drop of water touching it would simply have sizzled at once into steam.Slowly, the outside of the earth cooled off. Some scientists think that when the earth cooled enough, water vapor in the atmospherechanged into water and fell to earth as rain. Such a great rain would have lasted a very long time. Gradually, all the low parts of the surface would have filled up with water.

Other scientists think that as the earth'ssurface cooled, steam came rushing up out of volcanoes and geysers, just as it does today. When the steam hit the cool air, it was changed into water. The water then ran downhill. Slowly, over millions of years, it filled up the lowest parts of the eaarth's rocky crust. So, we don't know exactly how the sea was formed. But from what scientist have learned about the earth's rocks and its chemistry, they think the sea must have been formed in one of these two ways-or, perhaps, in a combination of both ways.