The Basilica Cistern, or Cisterna Basilica (Turkish: Yerebatan Sarnıcı – "Subterranean Cistern"), is the largest of several hundred ancient cisterns that lie beneath the city of Istanbul (formerly Constantinople), Turkey. The cistern, located 150 metres (490 ft) 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.
This subterranean cistern, in Greek kinsterne (κινστέρνη), was called Basilica because it was located under a large public square on the First Hill of Constantinople, the Stoa Basilica. At this location, and prior to constructing the cistern, a great Basilica stood in its place, built between the 3rd and 4th centuries during the Early Roman Age as a commercial, legal and artistic centre. The basilica was reconstructed by Illus after a fire in 476.