London could certainly be interesting. I'll need to look at the map of their metro system myself, as well as local fauna which may have mutated.
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EDIT: I've found a map which shows the lines that are in tunnels. From what I can tell, any lines with gray bars surrounding them are underground. [hider=Tube Map][img]http://oi65.tinypic.com/30uzlgg.jpg[/img][/hider]

For those who would like to see a geographically accurate (if a few years out of date) map, here's a pdf of one. [url]https://www.whatdotheyknow.com/request/224813/response/560395/attach/3/London%20Connections%20Map.pdf[/url]
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EDIT: Upon further inspection, most of the underground lines are barely below the surface. The Tube does have deeper lines but they appear to be only the newer ones.