### Salar de Uyuni (Salar de Tunupa)
Bolivia — Invisible underground whirlpools squeeze the crust of the world's largest salt flat into a flawless, repeating honeycomb grid.
For decades, scientists looking at the world's largest salt flat were stumped. Stretching for 3,800 square miles across an ancient Pleistocene lakebed, the blinding white salt crust is divided into a massive, repeating honeycomb grid of raised polygons. People used to guess the crust was just buckling from a lack of space, or cracking like dry mud.
But in 2023, fluid dynamicists finally cracked the case: the real architect is an invisible process called porous media convection. Beneath the hard salt crust is a deep layer of extremely salty brine. When the blazing sun evaporates water from the surface, the top layer becomes hyper-concentrated with heavy salt. That heavy water sinks, while lighter, fresher water rises from below to replace it.
This continuous loop creates thousands of invisible, circular underground whirlpools called convection cells. When these circular whirlpools press up against each other, they get squeezed into perfect hexagons, just like bees packing honeycomb to save space. As the heavy water sinks at the edges of the shapes, it dumps extra salt on the surface, physically building up the raised, repeating ridges across thousands of miles of desert.