STS058-88-17 Richat Structure, Mauritania October 1993 The Richat Structure, sometimes referred to as the Bull's Eye, can be seen in northwest-looking view. Contrary to a commonly believed notion, the structure was not made by the impact of a meteorite. The 24 mile (39km) wide structure was made by the erosion of layers of rock (limestone, sandstone, and shale) that domed upward. Geologists have not found any evidence of meteorite impact, surface volcanic activity, or salt. The best guess is that the structure was caused by deep injection of pre-volcanic melted rock (magma) that never quite ascended to Earth's surface. The depression or pit in which the domed rock rings sit is 328 feet (100 meters) deep from the surrounding plateau area (darker colored rock). The rock layers themselves are thought by many scientists to be several hundred million years old, and their variable resistance to erosion has produced a series of concentric ridges in the structure stepping down from the outside in. The structure is surrounded by fields of sand which is visible encroaching into the southern part of the structure. Small dry lakes or playas (whitish-gray color) are discernible to the east of the center part of the structure.