Bengboja

The Bengboja, or "Ice Fortress," is a structure developed by desert tribes of Da Therasti Okku to prevent ice from melting even in the fiery temperatures of the Great Equatorial Desert.   A bengboja is a round, cone-shaped structure with thick walls of brick and a special mortar that is completely impervious to water and keeps out heat, usually painted pure white, built atop an enormous, deep pit lined with clay and mortar. At the very top of the conical structure is a small hole that lets warmer air escape, while cool air enters at the base and circulates down to the bottom of the pit, where it is further cooled. The bengboja is also surrounded by shade walls to keep direct sunlight off of the structure, and usually also is connected to an underground water source (a spring or stream) that provides water for shallow evaporative cooling pools. Because of the way that the Bengboja is constructed, the cooling pools (Benglok) themselves are often covered in a thin layer of ice at night, which is harvested by workers and stored in the pit.   Bengboja are only constructed in desert areas with very low humidity, as humid air prevents the bengboja from functioning properly. Clear, cold desert nights fill the bengboja with cold, dry air, and the hot dry air of desert days enhances the evaporative cooling that keeps the temperatures deep in the pit very cold - near or even below freezing. Within the pit, layers of ice are separated by straw and wooden pallets, and the ice is stacked in such a way that any meltwater is directed away from the ice and drained into a cold water well below the pit that provides cold thermal mass to keep the pit cold. Overflow from the well is siphoned out to a drainage canal.