The Basilica Cistern is the largest subterranean water reservoir in the city. The entrance is the small building on the north of the Hagia Sophia Square. Built by Emperor Justin in 532 the cistern had been called as Basilica Cistern as it was located under the Stoa Basilica. The cistern is a gigantic underground chamber covering an area 140 m by 70 m. The cistern which is reached through a stairway of 52 steps contains 336 columns each 9 m high. The columns are arranged in 12 rows of 28 columns each spaced 4.80 m apart.