Salar de Uyuni is the worldâ€™s largest salt flat, spanning over 4,000 sq. miles (10,582 sq. km) in south-western Bolivia. It was part of a prehistoric salt lake, Lago MinchÃn. Unlike traditional deserts which are filled with sand, the Salar de Uyuni features vast expanses of glistening white salt. Salt water deposits seeped from the surrounding mountains. Since there is no outlet to the sea, they were deposited here which is the lowest point of the Altiplano plateau. That water then formed a giant lake. High salinity means that whilst this vast lake has long since evaporated under the sun, a thick crust of salt remains, forming what we now know as the Salt Flats of Salar de Uyuni. The Salar sits at approximately 3650 meters (11,975 ft) above sea level. There is no place else like it in the world.
The best time to go to Uyuni?
In the dry season, Salar de Uyuni is a special sight not to be missed. However, rain during February to April make it a totally different experience. As a thin sheet of water (a couple of inches) covers the flats, it reflects a perfect representation of the sky above so that it is impossible to tell where sky ends and the land begins. This mirror effect has given rise to spectacular photo opportunities.
source: https://www.boliviahop.com/discounts/salt-flat-tours-and-discounts/ , http://www.selectlatinamerica.co.uk/blog/50-random-interesting-facts-latin-america-probably-didnt-know/