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The Basilica Cistern is a fascinating ancient underground reservoir located southwest of Hagia Sophia in Istanbul. Built during the reign of Byzantine Emperor Justinian I (527-565), it is also known locally as the Yerebatan Cistern, referring to its sunken, marble-columned interior. The name “Basilica Cistern” comes from the basilica that once stood above it.

The cistern is massive, stretching 140 metres in length and 70 metres in width, covering an area of 9,800 square metres. Visitors access it via a staircase of 52 steps. Inside, you are surrounded by 336 columns, each standing 9 metres tall. These columns are arranged in 12 rows, with 28 columns per row. The columns, mostly made of marble, were taken from earlier buildings and feature a mix of Corinthian and Doric styles. Many of the columns are single pieces, but some are made from two sections.

The cistern’s brick walls rise 4.80 metres high, and the floor is covered with a thick layer of brick dust mortar to make it watertight. With a storage capacity of 100,000 tonnes of water, the Basilica Cistern once supplied water to the Great Palace of Constantinople and other buildings in the area.

One of the most intriguing features of the cistern is the two Medusa heads found at the northwest end, supporting columns. These heads are believed to be from the Roman period, though their exact origins remain unknown. Some historians think they were brought here to help support the columns, while others suggest they were placed as protective symbols. According to Greek mythology, Medusa was one of the three Gorgons, whose gaze could turn people to stone. The image of Medusa was often used to protect important buildings, adding an air of mystery and legend to the cistern.