An ab anbar (, literally "Cistern") is a traditional reservoir or cistern of drinking water in Greater Iran in antiquity. Structure To withstand the pressure the water exerts on the containers of the storage tank, the storage itself was built below ground level. This also provides resistance to earthquakes. Many cities in Iran lie in a region that has been affected by very large earthquakes. Since almost all ab anbars are subterranean structures capped barely above ground level, they inherently possess stable structures. The construction material used for ab anbars were very tough and extensively used a special mortar called sarooj which was made of sand, clay, egg whites, lime, goat hair, and ash in specific proportions, depending on location and climate of the city. This mixture was thought to be completely water impenetrable. The walls of the storage were often 2 meters thick, and special bricks had to be used. These bricks were especially baked for ab anbars and were called Ajor Ab anbari. Some ab anbars were so big that they would be built underneath caravanserais such as the ab anbar of Haj Agha Ali in Kerman. Sometimes they would also be built under mosques, such as the ab anbar of Vazir near Isfahan. The bottom of the storage tanks were often filled with metals for various structural reasons. The 18th century monarch Agha Muhammad Khan, is said to have extracted the metals from the bottom of the Ganjali Khan public baths to make bullets for a battle. Storage tank Some ab anbars had storage space tanks that were rectangular in design, such as in Qazvin, as opposed to cylindrical designs in Yazd. There were several designs for the arched roof of the storage spaces of each ab anbar, namely ahang, kalanbu, kazhāveh, or combinations of these depending on the features of the storage space. In the particular example of Sardar-e Bozorg ab anbar in Qazvin, the storage space was built so large that it became known as the largest single domed ab anbar of Iran. Doming the square plan was not an easy task, yet dome construction was not something new to these architects as is evident from the numerous domed masterpieces such as Soltaniyeh. Some sources indicate that the architects would first construct the storage space and then fill it up with hay and straw all the way up to where they could start constructing the