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 reserviors from far away sources through aqueducts.It is still possible to see remains of large aqueducts.Because Turks have always preferred running water,fter capturing the city from the Byzantines,they did not use cisterns probably.Most of them were usually converted into either small bazaars or storehouses.

The largest and most ornate of these cistern is YEREBATAN SARAYI. In its construction,columns and capitals of eralier temples were used and the 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.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 incidental protection against pollution.