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