What are the dark plains on the Moon’s surface called?

The dark, flat areas on the Moon’s surface are called maria (singular: mare), which is Latin for “seas.” Early astronomers thought they were actual oceans, but they are really ancient lava plains formed by volcanic eruptions billions of years ago. These maria cover about 16% of the Moon’s surface and are smoother than the surrounding highlands. The dark color comes from the type of rock found there, called basalt. Some of the most famous maria include the Sea of Tranquility, where astronauts from Apollo 11 first landed in 1969, and the Sea of Serenity.