The Basilica Cistern (Yerebatan Sarnıcı)

The Basilica Cistern, is the largest ancient cistern that lie beneath Istanbul (formerly Constantinople), in Turkey. The cistern, located 150 m west of the Hagia Sophia, was built in the 6th century during the reign of Byzantine Emperor Justinian I.
It was buit by Emperor Constantine and was enlarged by Emperor Justinian after the Nika riots of 532. 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 Topkapi Palace after the Ottoman conquest in 1453 and into modern times.
Located in the northwest corner of the cistern, the bases of two columns reuse blocks carved with the visage of Medusa. It is thought that the heads were brought to the cistern after being removed from a building of the late Roman period. Tradition has it that the blocks are oriented sideways and inverted in order to negate the power of the Gorgons' gaze.
A visit here is a cool and quiet break from the busy Istanbul streets.