The Basilica Cistern in Istanbul is an underground structure, commissioned by Emperor Justinian and built in 532. In Turkish language, it is known as Yerebatan Sarnıcı. It is the largest surviving Byzantine cistern in İstanbul. It was originally known as the Basilica Cistern because it lay underneath the Stoa Basilica, one of the great squares on the first hill.
The reason to build this structure was to service the Great Palace and surrounding buildings. It could store up to 80,000 cu meters of water delivered via 20km of aqueducts from a reservoir near the Black Sea, but was closed when the Byzantine emperors relocated from the Great Palace.