Yakhchāl is an ancient type of evaporative cooler.
Above ground, the structure had a domed shape, but had a subterranean storage space; it was often used to store ice, but sometimes was used to store food as well. The subterranean space coupled with the thick heat-resistant construction material insulated the storage space year round.
By 400 BCE, Persian engineers had mastered the technique of storing ice in the middle of summer in the desert. Ice was stored in a specially designed, passively cooled refrigerator.
This was a large underground space that had thick walls (at least two meters at the base) made out of a special mortar called sārooj, composed of sand, clay, egg whites, lime, goat hair, and ash in specific proportions.