In the arid deserts of Iran, as early as 400 BCE, Persian engineers mastered thermodynamics without electricity. The Yakhchal, meaning 'ice pit', was an ingenious evaporative cooling structure designed to manufacture and preserve ice through the sweltering summer months. The outer structure featured a tall, conical mud-brick dome made from Sarooj, a mortar composed of sand, clay, egg whites, lime, goat hair, and ash that was completely waterproof and heat-resistant.
Cold water was channeled into shallow aqueducts (qanats) running along the northern side of tall shading walls during winter nights. The night desert air cooled the water below freezing, creating thin sheets of ice that workers harvested and moved into the subterranean storage chamber. Deep underground, the Yakhchal's thick walls insulated the ice while subterranean channels pulled cool air down from tall windcatchers (badgirs) attached to the top of the structure.
Warm air rose and escaped through a hole at the peak of the dome, while subterranean drainage channels continuously drained away melted water to prevent it from degrading the remaining ice block. This combination of evaporative cooling, low thermal conductivity materials, and aerodynamic airflow allowed ice to remain frozen year-round in 40°C heat.