When we look towards the surface of the moon with the naked eyes, we observe some dark smooth regions. The earliest astronomers believed that these dark regions on the moon’s surface were covered with water like the seas on earth. It was Galileo who first studied these dark areas through his telescope in 1609 and concluded that these were the areas covered with water. Galileo also studied some bright rugged regions of the moon’s surface and found that the bright areas were highlands. The highlands catch the sun’s rays. The darker patches are low lying plain areas of moon. Therefore once these were thought to be seas of the moon.
In the later centuries, with the development of better telescopes, it became evident that the “Seas of the moon” were not seas at all, but low-lying and completely arid basins or plains. However, they continued to be called “Seas”. Astronomers called the dark areas Maria, meaning “Seas”. Strangely enough, no extensive Maria occurs on the lunar far side, which is almost entirely covered by mountainous terrae formations.
The Seas of the moon can be divided into two types: the circular shaped and the ones with irregular outline. The circular type Seas are generally surrounded by mountains. Mare Imbrium (Sea of showers) and Mare Crisium (Sea of crises) on the near side and Mare Moscoviense (Sea of Moscow) on the far side are of this type. The irregular seas, such as Mare Tranquillitatis (Sea of Tranquillity) and Oceanus Procellarum (Ocean of Storms) have no extensive bordering mountain walls.
The lunar seas are paved with successive lava flows. These might have been formed by the collisions of high velocity giant asteroids or meteorites. This would have ruptured the surface or triggered off volcanic eruptions allowing vast quantities of lava to well up from the lunar interior. The surface is made up of much the same elements as are volcanic rocks on earth but they contain no water. From the analysis of Maria basalts collected by the Apollo astronauts, it appears that the filling of the maria basins took place about 3500 million years ago. The terrae are much older regions than the maria. Most of these high ranges border the maria.