Located just south of the Hagia Sophia is the largest of several hundred cisterns beneath the city. Constructed in the sixth century during the reign of Justinian I (a Byzantine Emperor), the Basilica Cistern derived its name from the large public square on the first hill of Constantinople, the Stoa Basilica. Before there was a cistern on this spot, a Great Basilica was constructed here between the third and fourth centuries. Following several fires and civil unrests, the Basilica was removed and construction began on the cistern. Reportedly, 70,000 slaves were involved in the building of the cistern, which was to provide a water filtration system for the Great Palace of Constantinople and other buildings on the first hill. Surveys indicate that the cistern measures 453 feet by 212 feet (or 105,000 square feet) and can store upwards to 100,000 tons of water (although currently the capacity is kept at a much lower level).