The Basilica Cistern dates back to the Byzantine Empire, specifically the 6th century, and was constructed during the reign of Emperor Justinian. There are several cisterns (underground/covered reservoirs) underneath Istanbul, but the Basilica Cistern is the largest. It is supported by 336 marble columns 30 feet high and about 15 feet apart. Most of the columns are rather simple, with exception to the two columns supported by bases carved as Medusa’s head.