The Basilica Cistern, or Cisterna Basilica (, or , "Subterranean Cistern" or "Subterranean Palace"), is the largest of several hundred ancient cisterns that lie beneath the city of Istanbul, Turkey. The cistern, located southwest of the Hagia Sophia on the historical peninsula of Sarayburnu, was built in the 6th century during the reign of Byzantine Emperor Justinian I. Today it is kept with little water, for public access inside the space. History This subterranean cistern was called Basilica because it was located under a large public square, the Stoa Basilica, on the First Hill of Constantinople. Prior to its construction, a great basilica stood on the spot. It had been built during the Early Roman Age between the 3rd and 4th centuries as a commercial, legal and artistic centre. The basilica was reconstructed by Illus after a fire in 476. Ancient texts indicated that the basilica contained gardens, was surrounded by a colonnade and faced the Hagia Sophia. According to ancient historians, Emperor Constantine built a structure that was later reconstructed and enlarged by Emperor Justinian after the Nika riots of 532, which devastated the city. Historical texts claim that 7,000 slaves were involved in the construction of the cistern. The enlarged cistern provided a water filtration system for the Great Palace of Constantinople and other buildings on the First Hill, and continued to provide water to the Topkapı Palace after the Ottoman conquest in 1453 and into modern times. The existence of the cistern was eventually forgotten by all but the locals who still drew water from it until, in 1565, the French traveller Petrus Gyllius left a record of it. Gyllius recorded being rowed in between the columns and seeing fish swimming in the water beneath the boat. Measurements and data This cathedral-sized cistern is an underground chamber approximately by – about in area – capable of holding of water. The ceiling is supported by a forest of 336 marble columns, each high, arranged in 12 rows of 28 columns each spaced apart. The capitals of the columns are mainly in the Ionic and Corinthian style, with the exception of a few Doric capitals with no engravings. One of the columns is carved with raised pictures of a Hen's Eye, slanted branches, and tears, and resembles the columns of the 4th-century Triumphal Arch of Theodosius I (AD 379–395), erected in the 'Forum Tauri' Square, today's Beyazıt Square. Ancient