In the area of the Blue Mosque and Hagia Sophia there is an underground site that was built during the days of the Byzantine ruler Justinian (r. AD 529–565). An aqueduct brought water from springs 12 mi. [19 km.] distant from the city of Istanbul to supply this huge cistern.
The cistern is larger than a football/soccer field (44 x 150 yrds. [70m. x 140m.]) in size and supplied water to the city of "Istanbul" for hundreds of years! The cistern has undergone numerous repairs and cleanings—the most recent in 1994. A scene in the 1963 James Bond movie From Russia with Love was filmed there!