Maria, which is Latin for "seas", are large, dark, plain-like areas on the Moon's surface. They are actually vast impact basins that were filled with lava billions of years ago.

Here's a breakdown:

* Formation: Maria formed when large asteroids or comets crashed into the Moon's surface, creating enormous craters. These craters were then filled with lava that flowed from the Moon's interior.

* Composition: The lava that filled the maria is primarily composed of basalt, a dark-colored volcanic rock.

* Appearance: Because of the dark color of the basalt, maria appear darker than the surrounding lunar highlands, which are made of lighter-colored anorthosite.

* Flatness: The lava flows smoothed out the crater floors, creating the flat, plain-like appearance that we see today.

* Relative Age: Maria are generally younger than the lunar highlands, meaning they formed later in the Moon's history.

Fun Fact: While they are called "seas," maria are completely dry and have no water. The name was given by early astronomers who thought the dark areas were bodies of water.