Salar de Uyuni in Bolivia is the world's largest salt flat — over 10,000 square kilometres of almost perfectly flat white salt, high in the Andes at around 3,600 metres above sea level.

It's what remains of prehistoric lakes that dried up thousands of years ago. The salt crust can be metres thick, and beneath it lie huge reserves of lithium-rich brine.

In the rainy season, a thin layer of water can cover the flat, turning it into a giant mirror. Clouds, sky and people are reflected so perfectly that the horizon vanishes and travellers seem to walk in the sky.

The flat is so smooth and large that it's been used to help calibrate satellites that measure the height of the Earth's surface.