The entrance to the Yerebatan Sarayı, which means sunken palace, is located close to the Divan Yolu opposite the Aya Sofya. The cistern with a dimension of 70 m wide and 140 m long, is the largest remaining Byzantine cistern in Istabul. It was constructed after the Nika Revolt in 532 AD by the Byzantine Emperor Justinian the Great. The complex was known in Constantinople as the Basilica Cistern. Throughout the Byzantine period, the cistern was used to store the 80,000 cubic meter of water supply of the Great Palace and the other buildings on the First Hill. After the conquest by Mehmet II, it continued to provide water to the Topkapi Palace and its gardens. The water was pumped through a system of nearly 20 km of aqueducts from a reservoir in the Belgrade Forest, near the Black Sea. Yerebatan Saray fell into disuse during the Ottoman period and was kept as a junk yard until it was cleaned and restored by the Istanbul Municipality in 1985-88.
The ceiling of the reservoir is supported by a forest of 336 marble columns, each 9 metres high. The columns are topped by mainly Ionic and Corinthian style capitals, with. One of the columns is carved with raised pictures of a peacock-eye, slanted braches, and tears. This column resembles the columns of the Triumphal Arch of Theodosius I from the 4th century (AD 379–395), that were excavated in 1965 at Beyazit Meydanı. The majority of the columns in the cistern appear to have been recycled from the ruins of older buildings. In the left corner there are two columns that are supported by the heads of Gorgons (or Medusa). One of the heads is upside down and the other on its side. These Gorgon heads and the two outside the Archeological Museum belonged to the Forum of Constantine.
Here are the photos of the Basilica Cistern, click on the thumbnails to see greater pictures