Why Is That Landscape Blindingly White and Utterly Flat? Water That Never Leaves

The requirement is a basin with no outlet to the sea, in a climate where evaporation exceeds the water arriving. Rain and rivers carry dissolved minerals into the basin, water leaves only as vapour, and the minerals cannot leave at all, so they accumulate over thousands of years. A shallow lake forms after rain and evaporates, depositing a layer of salt, and repeating that cycle builds a crust that can be many metres thick. The surface produced is extraordinarily flat, because water spreads out perfectly level before evaporating and deposits its load evenly, which is why the largest examples vary in elevation by less than a metre across areas of thousands of square kilometres.

The crust is not simply table salt and the composition varies systematically:

• •Common salt is usually the most abundant component
• •Gypsum and carbonates precipitate earlier, since they become saturated sooner
• •Borates accumulate where volcanic sources supply them
• •Lithium and potassium salts concentrate in the brine beneath the crust
• •The minerals separate into zones, with the least soluble deposited furthest out
• •Some basins are dominated by soda rather than by common salt

These surfaces support activities that suit nothing else. Salt has been harvested from them for thousands of years, by scraping the crust or by evaporating brine in shallow ponds. Lithium extraction has become economically enormous, since the brine beneath several South American basins holds the world's largest accessible reserves, pumped to the surface and concentrated in evaporation ponds over many months, which is now a significant geopolitical matter and a substantial local environmental one given the water consumed. The flatness makes them useful for land speed record attempts and for calibrating satellite instruments, since a surface of known flatness and known brightness is exactly what an orbiting sensor needs for reference.

These surfaces are dry for most of the year and not all of it, and the wet phase produces the effects the photographs are known for. Rain or river inflow spreads a shallow sheet of water across the flat crust, in places only a few centimetres deep over hundreds of square kilometres, and because the surface beneath is level and the water is still, it forms an almost perfect mirror reflecting the sky. That reflection is what draws photographers to the Bolivian example in particular. The flooding also redissolves the surface and redeposits it evenly on drying, which is what keeps the plain flat and what erases vehicle tracks each year. The polygonal cracking pattern that covers the dry crust forms as the last water leaves and the salt contracts.

These environments look sterile and are not, which turns out to matter scientifically. Salt-tolerant microorganisms live in the brine at concentrations approaching saturation, using specialised mechanisms to avoid losing water, and some colour the brine pink, which is why evaporation ponds photograph the way they do. Brine shrimp survive in several. Flamingos feed on the organisms in some basins and breed on islands within them, which is why the largest flocks occur in exactly these places. Seeds and eggs persist through dry years and hatch when rain arrives, producing sudden brief abundance. The organisms are studied intensively as models for how life might persist in comparable conditions elsewhere in the solar system.

A basin with no outlet in a climate where evaporation exceeds inflow accumulates every mineral the water carries in, building crusts metres thick and surfaces that vary by less than a metre over thousands of square kilometres. The deposits separate into zones by solubility. Salt has been harvested for millennia, lithium extraction from the brine beneath is now economically enormous, and salt-tolerant microorganisms colour the brine pink.