Ahhhhhh, of course. Silly me.
But a TBM is some VERY SERIOUS digging. Other than Cheyenne mountain--and how many of THOSE do we have? I'd hope lots but am skeptical we've been that diligent in our own preservation.
Hey, if its cool enough to dig a coal mine, nuclear waste storage shaft, or Petroleum Reserve storage cavern, its probably cool enough to dig a lot of stuff for the USAF that we don't hear about.
Like nuclear weapons, communications nodes, secure data storage complexes, heck, they might even be making sure we don't have a Dr. Strangelove "Mine Shaft Gap".