While they might look like bodies of water from Earth, maria are actually vast, dark plains of solidified volcanic basalt. They were formed billions of years ago by massive volcanic eruptions that filled ancient impact craters.

Here's why they appear dark:

* Composition: The basalt is darker than the surrounding lunar highlands, which are made of a lighter-colored rock called anorthosite.

* Smoothness: The maria are relatively smooth compared to the cratered highlands, causing less light to be reflected back to Earth.

So, even though they're called "seas" (mare is Latin for "sea"), they're actually dry, flat plains of volcanic rock.