Bolivia's Salt Flat Is the World's Largest Mirror

The Salar de Uyuni sits 3,656 meters above sea level on the Bolivian altiplano, the dried bed of a prehistoric lake that evaporated about 11,000 years ago. It covers roughly 10,580 km² — the area of Jamaica — and is crusted over with a salt layer between a few meters and ten meters thick. Underneath is brine laced with lithium: the salar holds about a quarter of the world's economically extractable lithium, a resource that has shaped Bolivian foreign policy for two decades.

What matters to physicists is the flatness. The Salar varies in elevation by less than one meter across its entire extent. NASA has used it since 1999 to calibrate the radar altimeters on the Jason satellite series, which measure sea level from orbit. A flat patch of Earth that large, still, and reflective, is extremely rare. The satellites bounce radar off it and trust the return to within centimeters.

In the wet season, roughly January to April, a few centimeters of water pool on the surface and don't drain. The salt crust is too flat for runoff, so the water just sits there, a continuous thin mirror reflecting the sky in every direction. Tourists photograph themselves floating on cloudscapes. At sunset the horizon disappears. The local name for this effect, espejo, is just the Spanish word for mirror.

The salt under the surface forms hexagonal tiles, visible when the water evaporates — convection cells driven by denser brine sinking and lighter brine rising, a slow-motion Rayleigh–Bénard pattern you can walk on. Every rainy season dissolves and re-forms them. It's a geological Etch-A-Sketch the size of a small country.