Salar de Uyuni is the world’s largest salt flat at 10,582 square kilometers (4,086 sq mi). It is located in southwest Bolivia, near the crest of the Andes and is at an elevation of 3,656 meters (11,995 ft).
It is covered by a few meters of salt crust, which has an extraordinary flatness with average altitude 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. It contains 50 to 70% of the world’s lithium reserves. The large area, clear skies, and the exceptional flatness of the surface make the Salar an ideal object for calibrating the altimeters of Earth observation satellites. [source]