What we call "seas" on the Moon are actually large, dark plains of solidified lava called maria (singular: mare). The name comes from the Latin word for "sea" because early astronomers thought they were filled with water.

Here are two examples of maria:

* Mare Tranquillitatis: (Sea of Tranquility) - where the Apollo 11 mission landed.

* Mare Imbrium: (Sea of Showers) - one of the largest maria on the Moon.