Where is the flattest place on Earth?

The Salar de Uyuni in southwestern Bolivia is the largest salt flat in the world and among the flattest natural surfaces on the planet. Covering more than 10,000 square kilometres, its elevation changes by only about a metre across the whole area, an extraordinary degree of flatness. It formed from prehistoric lakes that evaporated, leaving behind a thick crust of salt. When a shallow layer of water covers it after rain, the salt flat turns into a giant natural mirror, reflecting the sky so perfectly that the horizon seems to vanish. Because of its flatness and reflectivity, it is used to calibrate the altitude measurements of satellites. It also holds a large share of the world lithium reserves beneath its crust.