Dasht-e Lut is a desert in southeastern Iran where satellites have measured the hottest ground on Earth, 80.8 degrees Celsius, and where the wind has carved the clay into ridges that run parallel and unbroken for forty kilometres, a city of earth with streets between its walls. Those ridges are the kaluts, and they stand near the town of Shahdad on the desert's western rim, some of them tens of metres high, all of them aligned with the wind that made them; from the road you walk straight into the gaps between them, which run flat and straight like streets between walls of packed earth. The time to be there is sunset. The sun strikes the ridges from the side rather than overhead, every wall throws a shadow, and the whole field turns from pale grey to gold to red before the desert goes dark and cold.
Lut means empty, and the emptiness is the point: no plants, almost no animals, a landscape of clay, salt and sand under a sky with nothing in it. The fiercest heat is measured at Gandom Beryan, a black lava plateau in the north whose name means roasted wheat, after a tale that a load of grain left there once cooked in the sun. East of the kaluts the stony plain gives way to an ocean of sand, the Rig-e Yalan, where the dunes rise hundreds of metres and are climbed for the view across a sea of them. Shahdad sat on a spur of the Silk Road, and its restored caravanserai, a walled courtyard of arches and cells, is where traders and their animals rested before or after crossing the sands. On moonless nights in spring and autumn, the only seasons cool enough to camp out, the Milky Way spills unbroken over the kaluts with no lamp for many kilometres. The Lut is the hottest place on the planet, and at night it is one of the darkest.