The sandstone caves of Nottingham are a network of caverns and tunnels deep beneath the English city. There's currently a project underway to "map" these caves, and the results look more than a little familiar.
I never thought I would see the day where science, tourism and "noclip mode" intersected, but it appears that day has come. Honestly, it looks like an Oblivion dungeon down there. Throw in some rats and a giant crab or two and they'll be indistinguishable.
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[Nottingham Caves Survey, via Neatorama]