Istanbul Basilica Cistern: (Yerebatan Sarayı)

This cistern, located in Eminonu district next to Hagia Sophia. It was used before as water supply for the city during the reign of Emperor Justinian I and 6th century AC. Basilica cistern is and underground chamber of 138 x 64.6 meters. There are 336 columns most of which are topped with Byzantine Corinthian capitals.

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 suggested that the people who put them 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. 

In 1988 this cistern was restored and converted to a museum.