It is in the west of Hagia Sophia Square. It was built by Justinian (527-565), the biggest indoor Byzantine cistern in size of 140 to 70 meters. Total 336 columns, each 9 meters high, carries the cross-vaulted brick ceiling. The water stored here was brought from outside of the city by aqueducts and channels in Byzantine period.
The cistern is toured by scaffolds for being on the level of fish living in water. It is believed that the column processed as a tree body in the center of the cistern was brought from another structure.
Almost forty of the columns are left behind the curtain wall in the southwest of the cistern which was made in the 19th century.
Medusa heads excavated after the renovation in the southwest corner are also interesting. Two Medusa heads which are reverse and leaning are used as pedestals of two columns.