Salar de Uyuni (also referred to as "Salar de Tunupa") is the largest salt flat (dry lake bed) in the world, with an area of approximately of 10,582 square kilometres (4,086 sq mi). It is situated in southwestern Bolivia, within the Daniel Campos Province of the Potosí Department, near the crest of the Andes Mountains, at an elevation of 3,656 m (11,995 ft) above sea level.
The Salar was formed as a result of transformations between several prehistoric lakes that existed around forty thousand years ago but had all evaporated over time. It is now covered by a few meters of salt crust, which has an extraordinary flatness with the average elevation variations within one meter over the entire area of the Salar. The crust serves as a source of salt and covers a pool of brine, which is exceptionally rich in lithium. The large area, clear skies, and exceptional flatness of the surface make the Salar ideal for calibrating the altimeters of Earth observation satellites. Following rain, a thin layer of dead calm water transforms the flat into the world's largest mirror, 129 km (80 miles) across.