* Maria (Latin for "seas") are vast, dark plains of basalt rock. Basalt is a type of volcanic rock formed from cooled lava flows.
* These plains formed billions of years ago. During a period of intense volcanic activity, lava erupted from the Moon's interior, flowed across the surface, and solidified.
* The maria are relatively smooth and flat. This is because the lava flows smoothed out the lunar surface as they spread.
Therefore, the maria are a testament to a time when the Moon was a much more volcanically active body than it is today.
