Lunar Maria, also known as lunar seas, are large dark plains on the surface of the Moon. They are called “seas” because early astronomers mistook them for bodies of water due to their smooth appearance. In reality, Lunar Maria are vast basaltic plains formed by ancient volcanic activity on the Moon. How are Lunar Maria formed? More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.