Cistern Basilica is an ancient masterpiece of engineering that fits the grandeur of Nova Roma from a period of Byzantine Emperor Justinian I. As Istanbul has been besieged often in the past; it made it necessary to have numerous cisterns to provide water to the city in troubled times.
The cistern today is the most well-preserved one with an entrance that descends 52 steps down and the immense rectangular plan of 140 m. x 70 m. impresses the visitors at first sight. Brick walls and floor have been insulated with Khorasan mortar and the cistern has 100.000 tons of water capacity. The structure is supported by 336 columns collected from ancient buildings to which the enormous weight of the ceiling is transferred by cross vaults and arches. Two Medusa heads used as column bases are among the best examples of art from the Roman Age.
Cistern Basilica which looks like an enigmatic forest of columns is all the more mystical with its special lighting and has become a set for many movies and the part of Dan Brown's best seller book Inferno that is highly recommended.
Update 15/10/2016: Don't miss the movie Inferno as well: http://danbrown.com/#films/inferno/