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DENVER - Chances are you've heard it before. You've probably been asked about it by one of your friends or relatives: is there a city under Denver International Airport?

Conspiracy theories surrounding DIA have gone worldwide.

If you type in "DIA Conspiracy" into YouTube, more than 80,000 videos pop up, each with tens of thousands of views.

So 9NEWS wanted to find out what's really under DIA.

"There are so many conspiracy theories that exist about DIA," DIA Communications Director Stacey Stegman said. "Some people say that there is an underground secret government unit that is housed [under DIA]; that there are alien structures that are hidden; a number of different psychological warfare testing. People question the murals. People question the art. I've never seen a facility that has so many questions about it."

Ever seen the Close Encounters of the Third Kind?

"One of the good theories is that if you plug in the coordinates, the alien race gave to the humans in the Close Encounters of the Third Kind, that somehow that's exactly where DIA is," Stegman said. "The coordinates actually land about 51 miles northwest of the Airport. Close Encounters of the Third Kind came out in 1977, 16 years before the airport was built. There's no link between and this great Hollywood film unfortunately."

Now to the old baggage rooms. The baggage system is still in place, though no longer used. But of course, there are rumors about that too.

"Some have suggested that the old baggage bins were made just the right size so we could carry bodies in them," Stegman said.

That's also not true. They were used only to transport bags years ago.

People also have theories about the tunnels, specifically the height of the tunnels.

"The tunnels are so high because they were built to house the baggage system," Stegman said.

And of course, one of the most well known theories is that a city exists under the airport.

If you call the thousands of airport workers under DIA a city, then perhaps that theory has some truth to it.

"I think people would be very disappointed if they were to actually spend some time under DIA," Stegman said."We are under the foot print of 53 million people. It would be pretty difficult to hide any covert activity with the thousands of workers that are down here every day."

Aside from the random handprints on the walls, the alien an airport worker drew on a wall and the old baggage system, all that's under DIA are everyday airport operations.

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