Built to provide water for the city of Istanbul during the reign of Emperor Justinian I in the 6th century CE, the Basilica Cistern is the largest cistern in Istanbul. According to some historical texts, 7,000 slaves were involved in the construction of the cistern. It provided a water filtration system for the Great Palace of Constantinople and continued to provide water to the Topkapı Palace after the Ottoman conquest in 1453.
The large space is broken up by a forest of 336 marble columns, each 9 meters (30 ft) high. One of the columns, there is the engraving of a Hen's Eye, slanted braches, and tears. Ancient texts suggest that the tears on the column pay tribute to the hundreds of slaves who died during the construction of the Basilica Cistern. At the back of the Cistern, there appears an upside down Medusa head supporting one of the columns, which attracts much attention of the guests.
The Cistern has the capacity to store 100,000 tons of water, despite being virtually empty today with only a few feet of water lining the bottom, populated by a large number of well fed goldfish.
The Basilica Cistern is open every day from 09:00 hours to 18:30 and the entrance fee is 10 TL (~7 USD) for foreign visitors.