Basilica cistern: largest underground water basin in Constantinople.
When Byzantium, renamed Constantinople, became the main imperial residence in the Roman Empire, it soon had more inhabitants than it could supply with the water of its wells. So, large cisterns were built to store water that would otherwise flow to the sea. One of these was the Basilica Cistern or, as it is called today, Yerebatan Sarayı. It was rebuilt by the emperor Justinian (r. 527-565) after the Nika revolt (532).
The Basilica Cistern is a large, vaulted space; its roof rests on twelve rows of twenty-eight marble columns, which are about nine meters high. As the total surface measures 65 x 138 meters, the maximum capacity is almost 85,000 cubic meters, which was brought to this cistern from a well about twenty kilometer away through a new aqueduct, also built by Justinian. The water was used in the imperial palace (hence the name, "imperial cistern").
The 336 columns - 246 are still visible - were brought to the Basilica Cistern from older buildings. Probably, one of these buildings was the place where the two giant gorgo heads were found that are still in the cistern and support two columns. Their original site may have been the Forum of Constantine, where similar heads have been found.
It is not clear why they are here. What is certain, is that the ancient Babylonians and - taking an idea from the east - Greeks believed that gorgo's faces warded off evil. After all, their looks could kill. This may be the reason to place these ugly faces in the first building, but does not explain why they were brought to this cistern, or why one of them is tilted and the other is even put upside down. Putting upside down pagan statues is not unheard-of, though: in some churches, the first Christians made altars of older monuments in this fashion. The symbolism is obvious. However, a cistern is not a church.
On top of the Basilica Cistern was one of the porticoes along Constantinople's main road, the Mese.