Lunar Maria are large, dark plains found on the surface of the Moon. They were formed by ancient volcanic activity and are less cratered than the surrounding highlands, indicating they are younger in geological terms. The name "maria" comes from the Latin word for "seas," as early astronomers mistakenly thought they were actual seas. These plains cover about 16% of the Moon's surface and are primarily located on the side that faces Earth. Some of the most well-known maria include Mare Tranquillitatis (Sea of Tranquility) and Mare Serenitatis (Sea of Serenity). Their smooth, basaltic rock composition makes them distinct from the lighter, more rugged highlands.