Why Is That Desert Floor Flatter Than Anything Built by People? Water Made It

In a basin with no outlet, occasional rain and runoff collect at the lowest point and form a shallow temporary lake. Because there is no outflow, the water leaves only by evaporation, and as it goes it deposits everything it was carrying, which is fine clay and dissolved salts. The clay settles out of still water into a layer of remarkable evenness, since standing water finds one level everywhere. Repeating that over thousands of cycles builds a floor whose flatness is limited by the flatness of water itself.

The characteristic features follow from repeated wetting and drying:

• •A hard crust of clay and salt, pale and glaring in sunlight
• •Polygonal cracks where the drying surface contracted
• •Occasional low ridges pushed up where crusts collide
• •No vegetation across most of the surface
• •A soft sticky layer beneath the crust after rain
• •Sharp edges where the flat floor meets the surrounding slope

The flatness is exceptional enough to be exploited. The salt flats at Bonneville and the enormous surface at Uyuni in Bolivia vary in height by well under a metre across tens of kilometres, which is flatter than any surface humans construct at that scale. Uyuni is used to calibrate the altimeters of satellites in orbit for exactly this reason, since its elevation is known precisely and does not change. Land speed records are attempted on such surfaces because no other natural ground offers the distance without a bump, and aircraft have landed on them in emergencies.

The surface looks sterile and is not, which is one of the more interesting things about these basins. Salt-tolerant plants ring the margins in zones according to how much salt each can stand, producing visible bands of vegetation that map the chemistry. Brine shrimp eggs survive in the crust for years and hatch within hours of flooding, which supports migrating birds that arrive in enormous numbers during the brief wet period. Microorganisms living inside salt crystals photosynthesise through the translucent mineral. And certain flamingo species depend almost entirely on these basins for breeding.

The surface is treacherous in ways that are not obvious to somebody standing on it in dry weather. The crust may be a few centimetres thick over saturated mud, and a vehicle that breaks through sinks and cannot be recovered without equipment that is many hours away. Rain turns the whole basin into a shallow lake within hours and the mud remains impassable for weeks afterwards. Distances are badly misjudged because there are no features to give scale, mirages are constant, and there is no shade or water anywhere on the surface.

In a basin with no outlet, water leaves only by evaporating, depositing fine clay and salt in layers as even as standing water itself, which builds an exceptionally flat floor. The largest vary by well under a metre across tens of kilometres and are used to calibrate satellite altimeters. The crust can be thin over saturated mud, and rain makes the whole basin impassable for weeks.