Dasht-e Kavir is a large desert lying in the middle of the Iranian plateau. The desert is composed of mud and salt marshes (kavirs). Tens of millions of years ago, this region was occupied by a salt-rich ocean that surround a small piece of continent in what is now central Iran. As the ocean dried up, it left behind a layer of salt.
Over time, the layer of salt was buried under a thick layer of mud. But salt has a fairly low density, so it started pushing up through the overlying sediment and eventually, over millions of years, the salt broke through and formed domes.