A cistern is more like a tank for holding or storing water. The Basilica Cistern was built in the 6th century and has provided water for the Istanbul residents since then. It is located underground very close to the Hagia Sophia. More than anything, what amazes everyone including yours truly is the technology which was thought of way back in the 6th century when this was built.
The Basilica Cistern is a giant underground cistern built by Justinian in 532 to provide water to the city in cases of siege. A wooden walkway floats above the fish filled water and winds between the pillars. Lights and piped music add to the eerie atmosphere and the statues of Medusa are impressive. (Opening times are between 09:00 and 18:30. Entry is 10 TL for non-Turkish citizens.)