A large basin excavated by asteroidal or planetismal impact on a planetary surface. Several concentric rings of mountains surround the basin. The ratio of the diameters of adjacent rings approximate to √2:1. The minimum dimension for rings to occur is about 300 km on the Moon; elsewhere examples of up to 3000 km diameter are known (Valhalla basin on Callisto). The classic example is the lunar Mare Orientale, 920 km diameter, with three mountain rings. Typical relief is 3 km. The energies involved in excavating the basins are of the order of 1027 J.