A salt flat is formed when a pool of salt water evaporates, depositing salt as it does. This layer of salt builds up over time and seasonal flooding causes a very flat surface to form.
Salar de Uyuni in Bolivia, the largest salt flat in the world.
When covered in water, salt flats become the largest mirrors in the world.
Salt flats are commonly used to calibrate observation satellites, as they provide very large and flat areas (Salar de Uyuni has an area of more than 10 000 square kilometres and varies in height by less than one metre). The surface of salt flats are highly reflective and because they occur in desert areas there is usually very little cloud cover and very clear air.