The Basilica Cistern is the largest of several hundred ancient cisterns that lie beneath the city of Istanbul (formerly Constantinople), Turkey. The cistern, located South West of the Hagia Sophia on the historical peninsula of Sarayburnu, was built in the 6th century during the reign of Byzantine Emperor Justinian I.
This underground structure was known as the Basilica Cistern as its was built underneath the Stoa Basilica, a large public square on the First Hill of Constantinople. According to ancient historians, Emperor Constantine had already constructed a structure, which was rebuilt and enlarged by Emperor Justinian after the Nika riots of 532. It provided water 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.

This underground structure was known as the Basilica Cistern as its was built underneath the Stoa Basilica, a large public square on the First Hill of Constantinople. According to ancient historians, Emperor Constantine had already constructed a structure, which was rebuilt and enlarged by Emperor Justinian after the Nika riots of 532. It provided water 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 bases of two of the columns reuse earlier blocks carved with the head of a Medusa. They are located in the northwest corner of the cistern. The origin of the two heads is unknown, though it is rumoured that the heads were brought to the cistern after being removed from an antique building of the late Roman period. Another mystery is why one of the heads is upside down, while the other is tilted to one side. It is commonly accepted by scientists that they were placed that way deliberately.

The cistern was used as a location for the 1963 James Bond film From Russia with Love. In the film, it is referred to as being constructed by the Emperor Constantine, with no reference to Justinian. It s location is a considerable distance from the Soviet (now Russian) consulate, which is located in >Beyoglu, the newer "European" section of Istanbul, on the other side of the Golden Horn.

Basilica cistern  is a very authentic place and a must see in Istanbul. The cistern is located at the >Sultanahmet square, when you are about to reach the >Sultanahmet Square on the right bank you will see the basilica cistern. There is a small building next to the tram line, there leads the stairs down to the Basilica cistern. Basilica is open every day from 09:00 hrs to 17:30 and the entrance fee is 10 YTL (~7 USD or 5 euro) for foreign visitors.