After the Rain, Bolivia Gets the World's Biggest Mirror

High in the Andes of southwestern Bolivia there is a place where the ground and the sky swap places. For part of the year the Salar de Uyuni is a dazzling white plain of salt. Then the rains come, a thin layer of water settles on top, and the whole plain turns into a mirror. Clouds float below your feet as well as above your head, and the horizon seems to vanish.

The Salar de Uyuni is the largest salt flat in the world, covering roughly 10,582 square kilometres, and it sits at about 3,656 metres above sea level. It is the dried-out floor of prehistoric lakes. Over thousands of years the water evaporated and left its salt behind, spread into a crust so even that the height of the surface varies by less than one metre across the whole enormous area.

That astonishing flatness is what makes the mirror possible. During the rainy season, roughly from November to March, water gathers on the salt in a shallow sheet. On such level ground it does not run off or pool into deep puddles. It simply lies there, thin and still, and on a calm day it reflects the sky almost perfectly. The mirror can stretch for about 129 kilometres, which is why people call it the largest mirror on Earth.

When the water dries up, the salar becomes useful in a completely different way. Scientists who calibrate satellites need big, bright, even patches of ground that look the same year after year, so they can check that the cameras and instruments in orbit are measuring correctly. The dry salar is ideal: huge, very flat, highly reflective, and usually under clear, dry air with few clouds. The United States Geological Survey lists it as one of its calibration sites, and for checking satellite altimeters it is said to work about five times better than the open ocean.

The salar is not empty, either. Three species of flamingo live in the region, along with around 80 other kinds of bird, and local cooperatives still harvest salt from the crust, around 25,000 tonnes a year. Visitors drive out across the white in four-wheel-drive vehicles, and in the wet season they wade through the mirror, taking photographs in which they seem to be standing on the sky.

The two faces of the salar, mirror in the rainy months and measuring tape in the dry ones, both come from the same simple fact: it is extraordinarily flat, and it is made of something very bright.

We know how it behaves because it is watched closely: the survey that uses it for calibration records which months are dry enough to measure and which are too wet, and the wet months are exactly the mirror season. So if you ever see a photograph of people who seem to be walking on clouds, there is a good chance it was taken here, on the floor of a lake that dried up long ago and left a mirror behind.