Mare ( (singular: mare ) is the name for a great plain on the moon. Several such maria exist. They were formed by ancient volcanic eruptions. The plural form of mare is maria. The Latin word maria means seas. Because of their high amount of iron, they reflects less light than the highlands of moon. So, they appear dark to the naked eye. They are called sea because ancient astronomers looked at the Moon and thought they saw seas and oceans on the Moon. The maria cover about 16% of the surface of the Moon and most of them are in the side of the moon which is visible from Earth.