Seems more than a little far-fetched, but plausible I guess? I do like the general concept of the machines they're allegedly using. I want to see one disguised as a giant sandworm à la dune.
I doubt it would be happening to the degree that these guys seem to think though. Hundreds of underground military bases connected by trains that travel at unbelievably high speeds seems like it would involve a lot of personnel and would be rather difficult to keep under wraps?
And you'd think this technology would be cropping up elsewhere if it was possible, considering how much money there is to be made digging stuff out of the ground...