The Basilica Cistern is a gigantic underground cistern built in ancient Constantinople, the old Byzantine Empire's capital. It is one of the most spectacular landmark still visible today in Istanbul.
It is located west of St. Sophia, between the Mesa in the south and the Theotokos Church of Chalkoprateia.
The cistern was built by Emperor Justinian in the basement of a large porticoed building, the Basilikè, after the latter's fire during the Nika sedition in 532. Prokopius of Caesarea describes at length in the Buildings the reconstruction of the Basilica and the reasons for the construction of this vast cistern:
'Digging deep into this square and one of the porticoes, the one facing south, the Emperor Justinian created a reservoir suitable for summer containing the overabundant waters of the other seasons'...