So I got to thinking: wouldn't it be a cool vacation to do a LOTR (Lord of the Rings) hike? I mean, pick spots around the world that look like the Shire, the old forest, Bree, Rivendale, Rohan, even Mordor. Travel along them at approximately the same distance pace as in the book. Simulating wargs and ringwraiths optional.
Most of the environments are readily accessible right here in north America. The most difficult locale to simulate would be, of course, the Mines of Moria. In the novel, the Mines are excavated caves going under a mountain and are about 40 miles long.
I don't know of any man-made underground structure like that, with the possibility of NORAD. But even if it were unnoccupied, it would be far too small. Consider this description from a government website:
"The main entrance to the complex is approximately one-third of a mile from the North Portal via a tunnel which leads to a pair of steel Blast Doors each weighing 25 tons. Behind the 25-ton blast doors is a steel building complex built within a 4.5 acre grid of excavated chambers and tunnels and surrounded by 2,000 feet of granite. The main excavation consists of three chambers 45 feet wide, 60 feet high, and 588 feet long, intersected by four chambers 32 feet wide, 56 feet high and 335 feet long. Fifteen buildings, freestanding without contact with the rock walls or roofs and joined by flexible vestibule connections, make up the inner complex. Twelve of these buildings are three stories tall; the others are one and two stories."
Well, it won't do - it's only half a mile long! more to the point, it doesn't go all the way under the mountain. However, it does have a nifty analogue to the Hollin Gate, those 25 ton blast doors. I wonder if the password "mellon" opens them?
One could make due with natural caves. I found this wonderful resource which lists US and World caves by length. Unfortunately, it seems that most of these caves are protected by environmental advocates. Furthermore, many of them don't go under any mountains.
One last possibility exists, and it is most promising: abandoned transit tunnels. Train tunnels in the mountains would have the right naturalistic feel to them, but also abandoned subway tunnels under major cities (like the 8 mile abandoned subway line under Cincinnati). While one abandons the naturalistic element, one does get the added bonus of the possibile encounter with 'orcs' (street punks) or even a Balrog (use your imagination!).
Personally, I would hope that there are some good abandoned rail tunnels in the Rockies.
Mordor is particularly interesting to find an analogue. My vote is for the city of Wilmington. But again, you loose the naturalistic feel of the book. Besides, there should be an active volcano around there somewhere.