Lunar sea

lunar sea
A lunar sea is any flat, dark plain of lower elevation on the Moon. The term was erroneously applied to this feature of the lunar surface by telescopic observers of the 17th century; in actuality, a lunar sea (or mare) is a huge lava flow and does not contain any water. All lunar seas occur on the side of the Moon that always faces the Earth. They are the largest topographic features on the Moon and can be seen from the Earth with the unaided eye, appearing as dark patches on a paler background.