Salar de Uyuni: The world's largest salt desert and lithium reservoir surrounded by volcanoes

Salar de Uyuni is the largest salt desert on Earth, stretching roughly 4,000 square miles (10,400 square kilometers) across the Andean plateau. Uyuni is famous for its gleaming expanses of salt bathed in water, and for the mesmerizing honeycomb patterns found in the desert's driest corners.

Salar de Uyuni has an average elevation of 12,000 feet (3,660 meters) above sea level — but it wasn't always so. Around 5 million years ago, the region where the Andes mountains sit today was low-lying and the climate became arid, Sarah McKnight, an assistant professor of hydrogeology at the University of Dayton in Ohio, told Live Science.

Related: Andes region formed in 4 stages over the last 24 million years, new modeling study suggests

Over time, extreme heat and scarce rainfall caused prehistoric lakes in the region to evaporate, leaving behind crusts of sediment and salt. Tectonic processes and volcanic activity then pushed those crusts up onto a high plateau, where they remain to this day.

Compared with other salt flats like Salar de Atacama in Chile, which has a salt crust more than 3,300 feet (1,000 m) thick in some places, Salar de Uyuni is "pretty thin," McKnight said, with a crust just 10 to 33 feet (3 to 10 m) thick. How much salt accumulates on a salt flat depends on the geology of the area, McKnight said, and the tectonic plates beneath Salar de Uyuni are much less active than those beneath Salar de Atacama.

Salar de Uyuni's salt crust covers a layer of extremely salty water, or brine. But water also sits on top of the crust in some places due to a process called "freshwater upwelling," McKnight said.

The salt flat is surrounded by snow-capped mountains and volcanoes that supply melt water to the rim of the crust. The freshwater slips beneath the crust but immediately separates from the brine due to the difference in salinity. "Since freshwater is less dense than the brine, it's actually going to override or float, essentially, on top of the salt water," McKnight said.

Freshwater upwelling is when freshwater migrates up through the crust and springs to the surface, creating lakes on the salt flat. Salar de Uyuni has between six and eight such lakes, which are vital for Animals like flamingos that live in the desert, McKnight said. The size of the lakes varies depending on the season and on drought.