The Basilica Cistern with just a few feet of water. In old Roman days it would be filled to the ceiling - 20 feet. There are 336 columns holding up the ceiling. Supposedly part of the latest James Bond movie - Skyfall - was shot in here. We saw the movie, I don't remember the scene.
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I am awed that it was built in 600 AD - and used until "modern times". http://en.wikipedia.org/wiki/Basilica_Cistern
— Peter Hovell