One of the magnificent ancient buildings of İstanbul is the Basilica Cistern located in the southwest of Hagia Sofia. Constructed for Justinianus I, the Byzantium Emperor (527-565), this big underground water reservoir is called as “Yerebatan Cistern” among the public because of the underground marble columns. As there used to be a basilica in the place of the cistern, it is also called Basilica Cistern.
The cistern is a giant structure covering a rectangular area of 140 meters in length and 70 meters in width. Covering a total area of 9,800 m2, this cistern has a storage capacity of approximately 100,000 tons of water. There are 336 columns, each 9 meters high, inside this cistern, which is descended by a 52-step stone staircase. These columns, erected at intervals of 4.80 meters, form 12 rows of 28 columns each. Most of the columns, which are understood to have been collected from older structures, were carved from various types of marble and consisted of one piece and two pieces. The walls of the cistern, 4.80 meters thick, made of bricks and the brick-paved floor, were plastered with a thick layer of Horasan mortar and made water impermeable. http://yerebatan.com/