When astronomers first started looking at the Moon through their telescopes, they saw large dark areas which they thought were water. Therefore they named these areas Mare, which is Latin for "sea". In fact these Maria are not water, but volcanic lava.
Lunar Maria vary in size from 200 km to 1200 km wide and are between 500 km and 1500 km thick. Most of the Lunar Maria were created when meteors impacted on the Moon's surface, forcing lava to the top.
The rough age of Lunar Maria can measured by finding the density of craters in them. The higher the density, the older that particular area is. Ages range from one to 3.5 billion years.
The major Lunar Maria are: