Locally, its known as Yerebatan Sarnc, Turkish for underground cistern.
It is 140 meters long and 70 meters wide, and had a storage capacity of 100,000 tons.
The Medusa heads are used casually as supports under the two columns at the northwest edge of the cistern.
One of them is positioned upside down and the other is tilted to the side.
Their strange positioning and the mystery of their origin attracts the most attention from visitors.
Its said that the heads were so placed to counter the deadly gaze of Medusa.
The cistern was closed and forgotten.
Sometimes they even caught fish.
The cistern was opened to the public in 1987.
Photo credit:mtgf/Flickr
Another Medusa Head lying on its side.
Photo credit:Nick Wadge/Flickr
Photo credit:Dennis Jarvis/Flickr
Sources:Lonely Planet/yerebatan.com/Istanbul Trails
