During the Roman empire, modern-day Istanbul, Turkey would have been the site of Constantinople. Underneath this city, the Roman, and later the Byzantine Empire built an expansive system of cisterns to store drinking water for the residents.
The biggest of these underground storage reservoirs is the Basilica Cistern, and it is an amazing sight to see. Commissioned by Emperor Justinian I in 532 A.D., it is 140 meters long and 70 meters wide. Many of the columns throughout the cistern were salvaged from ruins around the area, then transported underground to hold the city above.
How the labor force was assembled to build this gigantic cistern isn't known for certain, but historical texts seem to incite that over 7,000 slaves were used to complete this underground labyrinth. 336 individual marble columns stand underground holding up the city above, each measuring 9 meters high.
Fifty-two stone steps lead down into the cistern which is surrounded by a 4-meter thick firebrick wall. Over 100,000 tons of water can be held underground here, although it is no longer used for water storage.
The expansive Basilica Cistern has undergone a few renovations in its history, with major ones being done in the 18th and 19th centuries. Some of the most notable architectural features found in the cistern are two gigantic carved heads of Medusa.
The origins of these heads are unknown, but it is believed that they were scrapped from buildings around the empire to be used as column supports.
After Istanbul fell to the conquest of the Ottoman empire in 1453, the cistern was largely forgotten about. Around 100 years later, it is rumored to have been found by a Dutch scholar who investigated the claims that locals were able to get water from buckets through holes in their basements.
After it was rediscovered, locals turned the historic cistern into a trash dump, but it was later cleaned up and restored. In 1987, the Basilica Cistern was opened back up to the public, and now serves a major site of attraction to the area.