The underground cistern or the Basilica Cistern is the largest Byzantine cistern located in the old city quarters close to Hagia Sophia across the tram rails. This is a magnificent structure lying beneath the ground some 8 to 10 ms depth. The dimensions: 70m * 140m * 10m. It can hold no less than 100 thousand cubic meter of water. The roof is supported by 336 columns arranged in 12 rows of 28 each. The water level in the cistern changed from time to time. The water was being distributed through the pipes at different levels in the eastern wall. At present there is half meter water supporting a huge number of fish which appear to have adapted to U/G conditions. It was built during the reign of Justinian I (527-565) to supply water to the palaces in the vicinity.
Visitors enter through a small building and leave the place at some 50m down the road by the tram rails. You can tour cistern at ease in some 20 minutes time.
The ground was cleared off unwanted material 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.
Be warned that in rainy days the steps leading down to walkways may be rather slippery.