When we look towars the surface of the moon with the naked eyes, we observe some dark smooth regions. The earliest astronomers beleived that the dark regions on the Moon's surface were covered in water just like our planet Earth. It was astronomer Galileo who studied these dark areas through a telescope in 1609 and concluded that these areas were covered in water. The bright rugged regions had bright areas which were highlands. The darker patches are low lying areas of moon. Therefore once these were thought to be seas of the moon.
In the later centuries, with the developement of better telescopes, it became evident that the seas of the moon were not seas but low lying and completely arid plains or basins. However they were still called seas. Strangely enough, no extensive seas occur on the lunar far side, which is almost entirely covered in mountainous terrain.
The seas of the moon can be divided into two types: the circular shaped and the ones irregular outline. The circular type Seas are generally surrounded by mountains and jagged peaks. Mare Imbrium, Crisium, Moscovienese Tranquillatatis and Procallarum are names of different moon seas.
The seas are bombarded with cooled lava by the moon's formation. There are no water in the seas says a recent study.