Basilica cistern: largest underground water basin in Constantinople.
When Byzantium, renamed Constantinople, became the capital of the Roman Empire, it soon had more inhabitants than it could supply with the water of its wells and the little river west of it. So, large cisterns were built. One of these was the Basilica Cistern or, as it is called today, Yerebatan Sarayı, which was rebuilt by the emperor Justinian (527-565) after the Nika revolt (532).
It is a large, vaulted space; the roof rests on twelve rows of twenty-eight marble columns, which are about nine meters high. As the total surface is 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 with a new aqueduct, also built by Justinian. It was used to provide water to 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. The original place may have been the Forum of Constantine, where similar heads were 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.