The largest and most magnificent covered cistern in the city is entered through a small building to the west of the Hagia Sophia Square. The ceiling of this forest of columns is made of brick and is cross-vaulted. The name of the cistern comes from a basilica that was once located nearby.
It was built during the reign of Justinian I (527-565) to supply water to the palaces in the vicinity. Its 336 columns are arranged in 12 rows of 28 each. The cistern measures 140 by 70 meters. The columns are topped with capitals, some of them plain, but most in Corinthian style. The water level in the cistern changed from season to season. The water was distributed through the pipes at different levels in the eastern wall.
The ground was cleared during the major restoration in 1987, and when over one meter of mud was removed, the original brick pavement and two marble Medusa heads at the base of two of the columns were revealed. The walkway that was constructed at that time enables visitors to walk around the cistern