The Basilica Cistern (Turkish: Yerebatan Saray? – “Sunken Palace”, or Yerebatan Sarn?c? – “Sunken Cistern”), is the largest of several hundred ancient cisterns that lie beneath the city of Istanbul (formerly Constantinople), Turkey. The cistern, located South West of the Hagia Sophia on the historical peninsula of Sarayburnu, was built in the 6th century during the reign of Byzantine Emperor Justinian I.This subterranean structure’s name derives from a large public square on the First Hill of Constantinople, the Stoa Basilica, beneath which it was constructed. According to ancient historians, Emperor Constantine constructed a structure which was later rebuilt and enlarged by Emperor Justinian after the Nika riots of 532. It provided water for the Great Palace of Constantinople and other buildings on the First Hill, and continued to provide water to the Topkapi Palace after the Ottoman conquest in 1453 and into modern times.