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Basilica Cistern, Istanbul
Image by eleephotography
The Basilica Cistern, the largest of several hundred ancient cisterns that lie beneath the city of Istanbul was built in the 6th century during the reign of Byzantine Emperor Justinian I.
Before being converted to a cistern, a great Basilica stood in its place that contained gardens, surrounded by a colonnade and facing Hagia Sophia
After a fire that destroyed the Basilica, 7,000 slaves were used to contruct the cistern. It then provided a water filtration system 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.