The term 'Dark Region of the Moon' refers to the MARE, which is a Latin word meaning 'SEA'. The MARE are large, dark, basaltic plains on the surface of the Moon that were formed by ancient volcanic eruptions. They are the most prominent features on the Moon's surface and cover about 16% of its total area. The MARE are believed to be the oldest features on the Moon, with some estimated to be over 3.5 billion years old.