During a period when electricity was only a thing for the Gods, around 400 B.C.E., in the hot-arid deserts of Iran where temperatures touched 40 degree centigrade, ancient engineers had found a way to keep their ice from melting. Two thousand years back, a cold storage facility was being used. The impressive thing about it – it was clean and sustainable technology.
By 400 BCE, Persian engineers had mastered the technique of using yakhchāls to create ice in the winter and store it in the summer in the desert.
In most yakhchāls, the ice is created by itself during the cold seasons of the year; the water is channeled from the qanat (Iranian aqueduct) to the yakhchāl and it freezes upon resting inside the structure.
The building allows cold air to pour in from entries at the base of the structure and descend to the lowest part of the yakhchāl, large underground spaces up to 5,000 m3 (180,000 cu ft) in volume.
At the same time, the tall conical shape of the building guides any remaining heat upward and outside through openings at the very top of the building, and through this active process the air inside the yakhchāl remains cooler than the outside.via(wikipedia).
The yakhchāl is built of a unique water resistant mortar called sarooj, composed of sand, clay, egg whites, lime, goat hair, and ash in specific proportions, that is resistant to heat transfer and is thought to be completely water impenetrable, this material acts as an effective insulation all year round. The sarooj walls are at least two meters thick at the base.