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I have visited this place multiple times and it is truly surreal experience. People jump from the top of the roof into the water as it is extremely deep there.
It used to be a mine operated by prison guards from nearby prison but unfortunately one of the water passages got broken during the night and it got filled up with water overnight. Everyone survived but the story goes that the last man did not get away from there with dry feet.
Edit: [Couple of pictures](http://imgur.com/a/G43yi) I found from my laptop from the place. Also there is a standing crane in the bottom of it, to give a sense how deep it really is.