After reading about Raven Rock a while back, I drove out and checked it out myself. (I live about half an hour away.)
Interestingly, I drove up or by three of the four entrance driveways, close to the staggered Jersey barriers, and was never challenged in any way. I didn't actually approach the main gate on foot, but I did get within about 20 feet of one of the other entrances.
Two of the entrances I found appeared to be normal country roads, but they just happened to terminate in razor-wire topped chainlink fences and U.S. government signage. At least one of two roads didn't even indicate "No Outlet." Around here, most entrances to roads which don't connect to another road have signs saying so.
I found the road which I guessed led to the fourth entrance, but felt that driving up to that entrance too might be pushing it.
I assume I'm now on some domestic terrorist watch list for my actions. (Maybe not for stumbling on one entrance, but almost certainly for visiting three entrances in a short period of time. Obviously I was probing their defenses.
Of course, the fact that I drove through Catoctin a week or two later probably didn't help either.
)
It's very interesting to look at the entrances, look at the mountain, and imagine that there's an entire city under there. There are a number of interesting articles (some from "legit" dead tree periodicals, others just electronic reports) about "Site R" (Raven Rock) and many of the other strategic and Continuity of Government sites which are now more widely known.
Edited to add: The problem with stuff from the web is that you tend to get goofy UFO hunters and Illuminati-nuts and Black Helicopter-spotters adding their own spin and embellishments to what might, in some cases, be a kernel of truth. Who knows if there's anything secret under the Denver airport? You'll never get any useful information from the bozos who think it's a secret landing site/concentration camp/staging ground for an alien invasion.
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-twency