This truly unique Istanbul landmark held reserves of clean drinking water. Maintaining this reserve was necessary for the city in case of drought or military action. The volume of the tank is huge: in use, the cistern contained up to 100,000 tons of water, and served as the main source of water for the central part of the city and the Emperor himself. Later, when the Turkish Padishas seized power, they took water from the structure to water Topkapi gardens.