It located nearby Hagia Sofia Museum and was built in the 6th century during the reign of Justinian I. They had more than 6,000 slaves who worked in the construction of cistern. Basilica Cistern supported a water filtration system for the Great Palace of Constantinople. It is 140 meters by 70 meters, about 9,800 square metres and has 100,000 cubic metres of water. The water of cistern was coming from Belgrad Forest which was 25 km far away.