*Underground Cistern* One of the major problems in ancient cities was how to ensure a water supply during seige. Istanbul possessed a series of large cisterns which were constructed with this in mind. Some were open and others covered in. The latter were either square or rectangular in plan, and were roofed over with brick arches and vaults, supported on stone piers, the Yerebatan Sarnıcı is one of the largest covered cisterns of the era in Istanbul. It is situated not far from the court of Haghia Sophia.

The district is named after it . In the Byzantine era, it was known as the "Basilica Cistern". First constructed dur­ing the reign of Constantine I (306-337), this underground cistern was restored and extended by Justinian (527-565). it is 141 mt. in length and 73 mt. in width. There are twelve rows of columns sup­porting the superstructure,  each with 28 columns, a total of 336 in all . The col­umns are 8 ms. in height and are sur­mounted by composite capitals. The cistern was renowned for the coldness of the water. A street of houses abutting the walls of Topkapi Palace beside the pre­sent Gülhane Park, whose houses were recently restored as tourist pension ­dwellings.