This interesting card, which I received from Gökçe, shows the Underground Cistern (also called the Basilica Cistern), which I had no idea about before. The first things that come into my mind when I think of Istanbul are the bridges connecting Europe with Asia and of course Hagia Sophia. But I know that it's an ancient city with lots of munuments that aren't so popular, but are definitely worth seeing. I know it's possible to go on a 4 days trip to Istanbul from Odessa (Ukraine) by ship, so maybe someday I'll try it. Of course, I would definitely prefer to go there by train, but ships are OK too :)
The Basilica Cistern (Underground Cistern) was built by Justinian I after the bloody Nika Revolt in 532, probably as an enlargement of an earlier cistern which was constructed by Constantine the Great. During the Byzantium Period, it was used as a reservoir for water storage for the Great Palace and other buildings in the First Hill. During the Ottoman Period, the water was used for Topkapi Palace and watering the gardens of it. However the cistern had its brighest days during the Byzantium Age.
The interior of Underground Cistern is excellent. It is 138 m, 452 ft long by 65m, 213 ft wide. There are 336 columns in the cistern in 12 rows. Most of the column capitals are either in Corinthian or Doric in style. At the far end of the Cistern, there are two heads of Medusa which are put upside down or side ways. The Medusa heads are taken from an ancient pagan site and the position in which they were placed suggests that the people who put them there, were Christians and did not want to remind a god from pagan period. The water inside the underground cistern is collected rain water. The carp in the water are decorative and an incidental protection against pollution. Some people even think that the Byzantines originally also raised fish in the cistern.