The yakhchal in Kerman is located about one and a half kilometres from the centre of the city. This cone-shaped building is about eighteen metres high. The massive insulation and the continuous cooling waters that spiral down its side keep the ice stored there in winter frozen throughout the summer. These ice houses used in desert towns from antiquity have a trench at the bottom to catch what water does melt from the ice and allow it to refreeze during the cold desert nights. The ice is broken up and moved to caverns deep in the ground where ambient low temperatures remain constant and allow the ice to remain in a frozen state. As more water runs into the trench, the process is repeated. Often seen around the ice houses and many of the homes in the desert are towers called badgirs or wind traps. Built of mud or mud brick, these badgirs, mentioned by Marco Polo, are square or round, but the operating principle is the same: to catch the slightest breeze in the vents at the top and to funnel the cooling air down through internal, vertically-placed wooded slats to the water or dwelling below. Alternately, the badgir can function as a chimney, expelling warm air to pull cool air out of a qanat (an underground stream).