As I mentioned, underground tunnels are a staple of conspiracy literature. Examples include a whole genre of underground tunnel theories involving Denver International Airport. See, for example, this thread on abovetopsecret and this youtube video. But it’s never made sense to me that the government (or anyone else) would build underground in such a public spot as an airport. True, DIA is next to Rocky Mountain Arsenal, which would seem to make it convenient for the government, but the arsenal happens to be a Superfund site, and it’s under much too much scrutiny for secret construction.
If one wished to create underground passages, wouldn’t one at least put the entrances in out of the way spots? And placing them near interstates only makes sense, like ports on a river. So, what better place to choose than a complicated overpass/underpass complex in the middle of nowhere? In fact, it seems to me I’ve run across quite a few overpasses that seem more convoluted than necessary. I wouldn’t be surprised if one or more are “haunted,” so to speak, by more than meets the eye. – Glenn
[NOTE: In a comment to this post, a leading spiritual figure in the "hollow earth" tradition suggests an alternative interpretation.]