I found this to one of the most interesting sites that I visited in Istanbul. From the outside it looks like a small, plain building on the side of the street, but once you go down the stairs inside, it opens up into an enormous column-lines cistern that is beautiful to walk through.
It's lit just enough so that you can walk around and see some of the designs carved into the columns, and the two large medusa heads hidden over in a corner (definately hunt them down!) The darkness lends to the ambience and mystery of the cisterns and makes it a great place to really feel you are in old Constantinople.
Look close enough and you can see there are carp in the water!
If you own or manage Basilica Cistern, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.