Salt flats are dried-up desert lakes. They form in closed hollows where rainfall can’t drain away. In a wet climate, a lake would form but, in a desert, the water is heated and evaporates into vapour faster than it is replenished by rain. The salt and minerals dissolved in the water are left behind as a solid layer. Some salt flats are massive. Bonneville Salt Flats in Utah, USA, were formed by the evaporation of an ancient lake as large as present-day Lake Michigan. They are flat enough to be used as a raceway for setting land-speed records.