In 400 BCE, while most of the ancient world was sweating through summer, Persian engineers were enjoying ice cream in the middle of scorching deserts. They accomplished this impossible-sounding feat using a Yakhchal, a brilliant piece of ancient green architecture that harnessed basic physics to create and store ice year-round. These massive, conical structures looked like alien beehives sprouting from the arid landscape, but inside lay an ingenious subterranean cooling matrix.
The secret to the Yakhchal lay in a combination of passive evaporative cooling, thermal mass, and windcatchers known as badgirs. Built with a specialized heat-resistant mortar called sarooj—a unique mixture of sand, clay, egg whites, lime, goat hair, and ash—the extraordinarily thick walls insulated the interior from the blazing sun. Below ground, a massive subterranean space extended up to 5,000 cubic meters.
Water was channeled into the structure from nearby mountains through underground aqueducts called qanats. At night, clear desert skies allowed heat to radiate away rapidly, freezing water in shallow channels inside or adjacent to the dome. During the day, tall windcatchers captured even the faintest desert breeze, channeling air downward over cool water channels to plummet subterranean temperatures while forcing warm air up and out through an apex vent at the top of the dome.
This continuous air circulation, combined with deep subterranean storage, kept temperatures below freezing throughout blistering summers. These ancient fridges preserved royal provisions and enabled the creation of faloodeh, a traditional Persian frozen dessert made of thin starch noodles steeped in rose water and lime juice.