A lunar mare is a large, dark plain on the surface of the Moon. These features were formed by ancient volcanic activity and are primarily composed of basalt rock. The term "mare" comes from the Latin word for "sea," as early astronomers mistakenly thought they were actual seas. Lunar mares cover about 16% of the Moon's surface and are mostly found on the side that faces Earth. Some of the most well-known mares include Mare Tranquillitatis and Mare Serenitatis. They are less cratered than the surrounding highlands, indicating they are younger geological formations.