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Touring the mysterious underground of Denver International Airport

DENVER -- Denver International Airport has been the subject of a lot of speculation over the years. The airport held a Facebook contest for conspiracy hunters to get a first-hand look down below.

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Airport spokesman Heath Montgomery has an explanation for just about everything below the airport.

“They think that we have an underground bunker and underground tunnels that go to NORAD and military installations,” Montgomery said. “But if you look around it's really just a lot of stuff to support the airport.”

Or is it? There’s a mysterious alien on the wall. Even Montgomery doesn’t know where it came from.

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“We don't know where the alien sketch came from. It's been here for years,” Montgomery said. “We presume somebody who works down there drew it.”

But there are some areas, like an underground rail system, inactive since 2010, that couldn’t be accessed for “security reasons.”

"There's actually another subtunnel system that goes under here that I know this is going to sound sketchy, but we can't actually go down there for security reasons,” Montgomery said. “They have cleared this area and they have locked it up and we're not allowed to go in there.”

There is also a workspace that does maintenance on the train system that couldn't be accessed.

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Montgomery credits the conspiracy theory craze, in part, to the airport’s massive size: 53 square miles.

“People were thinking there's got to be more to it. What are they possibly digging those tunnels for? And really it was just all about infrastructure,” Montgomery said. "And then the internet started becoming popular in the mid-'90s and we could not escape it.”