I'd go with "absurdity as a misdirection."
There's a pretty good argument to be made that there is an underground complex beneath DIA. Supposedly there are still dump trucks full of dirt being moved to a landfill 2 miles away, and its been well over a decade since they finished the official excavation there. There's even a few photos (of questionable location) that show some massive tunnels under construction. Couple that with the apocalyptic art throughout the airport, and I think a bunker is well within the realm of possibility. Continuity of government and all that.
But for some reason everyone jumps to aliens...
From my understanding, the underground tunnel's existence are not doubted or denied, it's their purpose that's 'debated'. DIA said they made the tunnels to house a central baggage routing system, but the system got scrapped at some point for some reason.
"Plausible deniability" is what theorists are going to claim...
<sigh>