Legend has it that there are space aliens and lizard people living in the underground tunnels at Denver International Airport (DEN). But if it’s true (and why not?), the reptilian and otherworldly beings will soon need to find a new place to hang out.

That’s because DEN airport is planning to repurpose some of its subterranean real estate into pedestrian walkways that can serve as alternatives to, and backups for, the airport’s troubled train system.

At DEN airport, trains connect the main Jeppesen Terminal to concourses A, B and C.

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Passengers may skip the train and instead stroll or ride moving walkways from the main terminal to Concourse A. But the train — officially called the Automated Guideway Transit System — is the only transportation option for getting between concourses A and B and between concourses B and C.

The original circa-1995 train system is currently undergoing a much-needed $75 million upgrade as part of the DEN’s “Vision 100” strategic plan to serve 100 million annual passengers in the next several years.

Improvements include 16 new train cars and the replacement of aging infrastructure that is prone to malfunctions. The glitches sometimes last just a few minutes, but as recently as May 2026, mechanical problems with trains forced the airport to deploy shuttle buses to move passengers between concourses.

Train to the Gates Updates: Crews have repaired the mechanical issue and trains are now fully operational. Shuttle buses from Concourse A to Concourses C are also running to help move passengers while the train operation returns to normal service. https://t.co/BZRJheqi7V

— Denver Int’l Airport (@DENAirport) May 6, 2026

Although DEN’s records show that the airport trains run glitch-free more than 99% of the time, even short outages create stress, platform gridlock and missed flights “simply because we have so many people going through our airport,” Jim Starling, DEN’s chief construction and infrastructure officer, told TPG.

Finding an alternative to DEN’s trains

Installing ziplines between concourses as alternatives to the train sounds fun but is sadly impractical. Connecting all the concourses with bridges was considered but rejected due to time and cost.

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Instead, during planning workshops, airline and DEN officials determined that the best solution was to repurpose portions of the airport’s existing underground baggage tunnels into pedestrian walkways. Those tunnels were originally built for the airport’s ill-fated automated baggage system, whose technical failures delayed DEN’s planned 1993 opening by 16 months and left sections of the tunnel network largely unused for decades.

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In a statement announcing the underground walkway plan, Denver mayor Mike Johnston called it “a big win for Denver’s travelers.” The tunnel transformation also got thumbs up from United Airlines (Denver’s largest airline customer), American Airlines and Southwest Airlines (which counts DEN as its largest operation in terms of flights).

“The addition of pedestrian walkways at DEN is a significant investment and will give our customers more options for their connecting flights,” Jonna McGrath, United’s vice president of airport operations, said.

Lisa Hingson, vice president of customer experience and innovation at Southwest, said the new pedestrian walkways would be “a tremendous addition” to recent enhancements such as TSA PreCheck Touchless ID and Touchless ID self-bag drop. “The ​addition of ​pedestrian walkways adds flexibility ​and reliability ​for ​our customers and improves operational ​resilience,” ​Amanda ​Zhang, American Airlines’ vice president ​of corporate ​real estate, ​said.

Making it happen

The tunnels to be converted are wide enough for two-way pedestrian traffic and currently contain some of the equipment from the old, abandoned baggage system. So that will need to come out.

“If you go down there today, what you’ll see is a lot of concrete,” Starling said. “And that’s not the environment we’d want to have for people to walk through.”

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Making pedestrian walkways out of tunnels built to move baggage would entail upgrading the floors, adding walls and appropriate HVAC systems, and possibly installing moving walkways, Starling added.

And then there’s the challenge of what Starling termed “vertical circulation” — the tricky job of getting passengers down to the tunnels from one concourse and then back up at another.

Timeline and budget

DEN airport estimates the cost of creating pedestrian tunnels at DEN to be between $300 and $700 million.

“That’s a wide range,” Starling said, “but it reflects the fact that we are at the concept level.” Once design plans are finalized, construction of the tunnels could begin as early as 2027.

And what about the lizard people?

Over the years, DEN has neither confirmed nor denied rumors of secret Illuminati, outer space aliens and, yes, lizard people making their homes in the airport’s underground tunnels. Instead, the airport has good-naturedly leaned into the mysteries and conspiracy theories with exhibitions such as “Conspiracy Theories Uncovered.”

Johnston is happy to play along. In the announcement of plans for the pedestrian walkways at DEN, he said: “And who knows… maybe along the way, travelers will finally get a closer look at the underground tunnels and decide for themselves what’s fact and what’s fiction.”

The global discount supermarket chain Aldi recently announced plans to expand into the Colorado market, opening 50 stores within the next two years. According to Denver’s Permitting and Licensing Center, the grocery chain has applied for permits at two locations in the city.