Located on the historic Sultanahmet peninsula, close to Hagia Sophia and Sultan Ahmed Mosque (“the Blue Mosque”), the Basilica Cistern is an impressive underground structure built at the time of the Byzantian Emperor Justinian in the sixth century. Underground cisterns were commonly built in the middle ages to collect and store rainwater. There were possibly hundreds of them in Istanbul. The Basilica Cistern was by far the largest, measuring approximately 170 by 70 meters and supported by 336 marble columns mainly with Corinthian and Doric style capitals. The columns are 9 meters tall and most of them are thought to be taken from older buildings. There are also two unusual column bases in the cistern with Medusa heads carved on them. The blocks stand upside down and sideways, possibly for practical reasons. They are nevertheless believed to protect the cistern.
The Basilica Cistern was named after Basilica Stoa that once stood above ground in its place. The cistern underwent two major restorations during the Ottoman period in 18th and 19th centuries. Modern repairs and restorations continued in modern day Turkey and the cistern remains open for visitors.