* Maria are vast, dark plains of solidified lava. They are called "maria" (Latin for "seas") because early astronomers thought they were bodies of water.
* Volcanic eruptions filled impact basins. The maria formed when large asteroids or comets struck the Moon, creating huge impact basins. These basins were then filled with molten lava that erupted from the Moon's interior.
* Evidence of volcanic activity. The maria contain volcanic features like shield volcanoes, lava flows, and rilles (canyons carved by flowing lava).
So, while the Moon is currently geologically inactive, the presence of maria is a clear sign that it once had a period of significant volcanic activity.
