The Basilica Cistern is the largest of several hundred ancient cisterns that lie beneath the city of Istanbul (formerly Constantinople), Turkey. The cistern, located 500 feet (150 m) southwest of the Aya Sophia on the historical peninsula of Sarayburnu, was built in the 6th century during the reign of Byzantine Emperor Justinian I.

The name of this subterranean structure derives from a large public square on the First Hill of Constantinople, the Stoa Basilica, beneath which it was originally constructed. Before being converted to a 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 Ilius after a fire in 476.

Ancient texts indicated that the basilica contained gardens, surrounded by a colonnade and facing the Aya Sophia. According to ancient historians, Emperor Constantine built a structure that was later rebuilt 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 Topkapi Palace after the Ottoman conquest in 1453 and into modern times.

The Basilica Cistern and the Medusa

There are two Medusa heads in the northwest end of the Basilica Cistern. These Medusa heads are considered as one of the sculpture masterpieces of the Roman Era. And their origin is not known. The legend has it that Medusa had been a very beautiful young woman. Her beauty had taken the attraction of the gods. And, Athena the goddess, the most favorite daughter of Zeus, had been very jealous of her. The God of the seas, Poseidon had been an admirer of Medusa as well. One day, Poseidon had forced Medusa to make love to him in the temple of Athena.

Athena finding the situation humiliating for herself had punished Medusa by turning her into a six-teeth monster. Then she had found her punishment insufficient, collaborated with Perseus to have him cut the head of Medusa who had been a human, consequently a mortal. As Perseus cut Medusa’s head, a horse with wings named Pegasus had been born from the blood right there. It had rumbled and flown to the sky. In ancient times, it had been believed that Medusa had turned people staring her into stone. Because of this, the great buildings and sacred places had been protected with the Medusa figures.