Much of the Fallout 3 environment relates to the actual area surrounding Washington, DC. It seems that a prominent natural cavern would be based on an actual location as well. I’m planning on visiting the DC/Maryland area later this year and have been wondering if a real location would be worth the side trip.
There are several cave systems in Virginia within a 2 to 4 hour drive of DC. Check out a list of them here: http://en.wikipedia.org/wiki/List_of_caves_in_the_United_States
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