The largest and most magnificent covered cistern in the city is entered through a small building to the west of the Hagia Sophia Square. The ceiling of this forest of columns is made of brick and is cross-vaulted. The name of the cistern comes from a basilica that was once located nearby. It was built during the reign of Justinian I (527-565) to supply water to the palaces in the vicinity. Its 336 columns are arranged in 12 rows of 28 each. The cistern measures 140 by 70 meters. The columns are topped with capitals, some of them plain, but most in Corinthian style. The water level in the cistern changed from season to season. The water was distributed through the pipes at different levels in the eastern wall.

The ground was cleared during the major restoration in 1987, and when over one meter of mud was removed, the original brick pavement and two marble Medusa heads at the base of two of the columns were revealed. The walkway that was constructed at that time enables visitors to walk around the cistern.

Medusa
It is not known exactly where these two heads came from, but there is a rumour saying that they were brought here after being removed from an antique building of the late roman period. Another mistery is about why one of the Medusa's heads is upside down while the other is tilted to one side. It is commonly accepted that they were placed in that way deliberately.
It is said that Medusa was one of the three underground Gorgona Giant Sisters. Out of the these three sisters only Medusa was mortal and she had the power of transforming people who looked at her into stones. It is said that in the old times the statues and pictures of Medusa were placed in very important buildings and private places to keep them away from bad omens.
In another rumour, Medusa is said to be a girl who was very much proud of her black eyes, long hair and beautiful body. For a long time she was in love with him and she became jealous of Medusa's love with Perseus. For that reason she turned Medusa's beautiful hair into horrible snakes. Then whoever looked at Medusa turned into stone. When Perseus saw that Medusa was bewitched and she was turning people into stone by a mere glance, he but here head, then won many wars by showing his beloved one's head to enemies. From that time onwards it is said that byzanthine swords' handles were stylised with Medusa's head.