Is the largest of several hundred ancient cisterns that lie beneath the city of Istanbul (formerly Constantinople), Turkey. The cistern, located 500 feet (150 m) southwest of the Hagia Sophia on the historical peninsula of Sarayburnu, was built in the 6th century during the reign of Byzantine Emperor Justinian I. Historical texts claim that 7, 000 slaves were involved in the construction of the cistern. The enlarged cistern provided a water filtration system for the Great Palace of Constantinople and other buildings on the First Hill, and continued to provide water to the Topkapi Palace after the Ottoman conquest in 1453 and into modern times. The cistern has the capacity to store 100, 000 tons of water, despite being virtually empty today with only a few feet of water lining the bottom, it is home to large numbers of.