The Basilica Cistern is one of many cisterns that lay beneath the city of Istanbul. Built between the 3rd and 4th centuries, it provided water for the Topkapi Palace. Right around the 14th-ish century, when the Ottomans conquered the city, the cistern was lost. It wasn't rediscovered by the city at large until about a hundred years later people were found drawing up water and fish from their basements.
There's still plenty of water in the cistern, water and fish. Tourists walk around on raised platforms. It's very slippery so I, with my penchant for falling, treaded very slowly. It was very cool in a damp, drippy, eerily lit way. The space isn't very large but the many columns, the dark, and the lighting made the place seem vast and unending.
Amongst the columns are two two Medusa heads. No one is really sure why they're there or why one is on its side and the other upside down. I think the favorite theory is that a couple columns needed repair and the Romans, not caring about Medusa, took the heads from a temple to shore up columns and were placed not right side up because that's just how they fit the best.
I tried to get some pictures of the fish as well but with them moving about (as fish do) and the weird lighting it did't work out so well for me.
Can you just imagine though...you're thirsty so you grab your rope and bucket, go down to your basement, draw up some water and oh there's dinner too. Shudder.