The dark spots you see on the moon are actually giant, solidified lava fields called maria. These were formed billions of years ago when huge amounts of liquid lava gushed out of the moon's interior and filled large craters after meteorite impacts.

Composition: The fields are mainly composed of basalt, a dark, iron-rich volcanic rock similar to that found on the ocean floor here on Earth.

Volcanic activity: It was long thought that volcanism on the moon died out billions of years ago, but new analyses of lunar samples suggest that there may have been active volcanoes as recently as 120 million years ago.

Lava tubes: Beneath the surface are hollow lava tunnels, or lava tubes. These giant caverns are considered ideal locations for future lunar bases because they protect against radiation and extreme temperatures.

Asymmetry: There are significantly more and larger lava fields on the side of the moon that faces Earth than on the far side, which is because the crust is thinner on the front side.