Istanbul was one of the most often besieged cities in the world and has always needed permanent water supplies. And as a result many underground cisterns were built during the Byzantine Empire. Water was brought to these big reservoirs from far away sources through aqueducts. It is still possible to see remains of a large aqueduct in Unkapani. This is called Bozdogan Kemeri (Aqueduct of Valens) and was built in 375 AD by the Emperor Valens. Because Turks have always preferred running water, after capturing the city from the Byzantines, they did not use cisterns properly. Most of them were usually converted into either small bazaars or storehouses. The largest and most ornate of these cisterns is Yerebatan Sarayi. In its construction, columns and capitals of earlier temples were used and this provides a very decorative appearance. This is why it is called saray which means “palace” in Turkish.
Yerebatan Sarayi was dug and built probably after 542 by Emperor Justinian I. There are 336 columns most of which are topped with Byzantine Corinthian capitals. The cistern is 70 m / 230 ft wide and 140 m / 460 ft long.
Between 1985-1988, the Municipality of Istanbul cleaned and restored it thoroughly and built a wooden walkway between the columns. In addition to that there are special effects presented by a light and sound show. By looking at the water level marks on the plaster walls which reach the height of the capitals, it is possible to understand that the cistern was very full in times gone by.
Two Medusa heads were used to form bases for two columns in a far corner of the cistern. The position in which they were placed suggests that the people who put them there were Christians and did not want to revere a god of a 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.Yerebatan Sarayi (Basilica Cistern), 6C AD, Istanbul