A large impact crater within which one or more concentric inner rings of mountains are present. There are three morphological types: peak–ring basins, in which there is a single inner ring; central peak basins, in which both an inner ring and a central peak are present; and multi-ringed basins, in which there is more than one inner ring. The rings are thought to form when the central region of the crater floor rebounds upwards and then collapses again, producing the first inner ring. In larger craters the central region may continue to oscillate up and down, forming additional rings. As many as six concentric rings have been noted in some basins. The Orientale Basin on the Moon is a beautifully preserved multi-ringed basin 930 km in diameter. The Moon’s Imbrium Basin, nearly 1300 km across, is larger, but much of it has been covered by younger lavas. The South Pole Aitken Basin, 2400 km wide, is the largest lunar impact structure of all. The largest basin in the Solar System is Valhalla on Callisto, 3000 km across. The Hellas basin on Mars (see Hellas Planitia) is 2300 km in diameter, but this has been heavily eroded, and its floor is covered in sediments. The size above which inner rings start to form varies from planet to planet, and also between different regions of the same planet. The smallest basins on the Moon are about 140 km in diameter, but on Mercury basins as small as 100 km are found.