The coolest place! The largest of several hundred ancient cisterns that lie beneath the city.

Emperor Constantine constructed a structure which was later rebuilt and enlarged by Emperor Justinian after the Nika riots of 532. The enlarged cistern provided water 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.

This cathedral-sized cistern is an underground chamber approximately 143 metres (470 ft) by 65 metres (210 ft) - about 9,800 square metres (105,000 sq ft) in area - capable of holding 80,000 cubic metres (2,800,000 cu ft) of water. The ceiling is supported by a forest of 336 marblecolumns, each 9 metres (30 ft) high, arranged in 12 rows of 28 columns spaced 4.9 metres (16 ft) apart. The capitals of the columns are mainly Ionic and Corinthian styles, with the exception of a few Doricstyle with no engravings.

The cistern is surrounded by a firebrick wall with a thickness of 4 metres (13 ft) and coated with a waterproofing mortar. The cistern's water was provided from the Belgrade Woods—which lie 19 kilometres (12 mi) north of the city—via aqueducts built by the Emperor Justinian.