Moon surface features, artwork. The surface of the Moon is divided into two main regions: bright mountainous highlands (upper frame) and darker, lower-lying seas (or maria, filling most of the frame). The seas are huge impact craters that filled with lava early in the Moon's history. The surface of the Moon is covered with craters produced by meterorite impacts. It also has volanic craters (one sectioned at lower left). A rille is seen running from centre to lower right, through an earlier impact crater. A rille is a long depression formed when a section of the crust sinks between faults (seen where the rille is sectioned at right).