For years, Instagram made us believe that Bolivia’s Salar de Uyuni was Earth’s biggest mirror. Visitors post dazzling photos of clouds, mountains, and themselves doubled against the thin sheet of water covering the salt desert. The images went viral. Tourists flocked to capture their own illusions of walking in the sky.

But science, it turns out, tells a more complicated story. Indeed, part of the Salar acted like a perfect mirror, but only some of the time. But for the entire Salar desert area to be a perfect mirror, the entire area should have backscattered light, which it doesn’t.

A Desert-Sized Mirror With an Expiration Date

A new study in Communications Earth & Environment overturns the idea that the 10,000-square-kilometer Salar is a permanent reflector. As lead author Stefano Vignudelli and colleagues write, “The Salar de Uyuni is not a vast uniform mirror for the radar altimeter. Therefore, it is also likely that it is not a large mirror for optical wavelengths.”

The team analyzed nearly 400,000 radar measurements from Europe’s Sentinel-3 satellites, collected over eight years. These instruments fire pulses toward Earth and measure how smooth the surface is by the strength of the returned signal. A true mirror needs to be almost impossibly smooth — within fractions of a micrometer.

To verify satellite data, the researchers staged an ambitious field trip to the salt desert’s inaccessible interior in February 2024, right when a Sentinel-3 passed overhead. They found the Salar reflecting the sun as a bright white disc, “a mirror image of the sun,” the paper reports. A drone photograph confirms this.

That may sound like this validates the claim of the Salar being a giant mirror. The only problem is that this effect is fleeting. The smooth, reflective surface appears mainly in the heart of the wet season, from late January to early March. Outside of those months, the salt flat looks more like cracked earth or shallow puddles.

Why the Mirror Comes and Goes

The Salar’s mirror effect depends entirely on rainfall and evaporation. With enough water, the desert transforms into a giant shallow lake a few centimeters deep. “The surface starts becoming radar smooth at the beginning of the wet season in December,” the researchers note, with the best conditions in February.

But the illusion fades quickly. Strong evaporation in the high-altitude desert sucks water away at a rate of about two millimeters per day. By April, the smoothness vanishes. Between 2016 and 2024, mirror-like conditions outside the rainy season appeared only once, after an unusual storm in August 2018 left the surface coated with 1.4 centimeters of water.

Interestingly, the Salar defies expectations when it comes to wind. You’d think that breezes would ripple the shallow water and ruin the mirror. But they don’t. Because the water is so thin, even gusts fail to make waves. Physicists suspect there’s a threshold wind speed below which waves cannot form in such shallow conditions.

Climate, Tourists, and Satellites

This shimmering desert mirror is not just an Instagram playground. It’s also a climate proxy. Rainfall in Bolivia’s Altiplano is shaped by El Niño and La Niña cycles, but in recent years, those patterns have grown less predictable. “Such discrepancies highlight the complexity of ENSO’s influence on the Altiplano’s hydrology,” the study explains. That means future visitors might find the mirror harder to catch as climate change reshapes rainfall.

The work also highlights how satellites are becoming essential tools for more than just weather or oceanography. Radar altimetry, usually used to study sea levels, proved perfect for detecting smoothness on a desert plateau. The scientists suggest that “a swarm of small satellites” could one day provide more frequent snapshots of the Salar’s shifting surface.

So the Salar de Uyuni is not a permanent mirror. It’s a seasonal performance, one staged by water, salt, and climate. If you want to see yourself walking in the sky, you’ll need luck, timing, and maybe a good tour guide willing to drive deep into the flooded flats.

For now, the takeaway is simple: the world’s “largest natural mirror” is real, but it comes with an expiration date.