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DENVER -- Downtown Denver is full of colorful characters and colorful places, but there’s much more to the city’s past than what meets the eye.

Denver has a treasure trove of history underground.

Colorado author Tracy Beach literally wrote the book on Denver’s tunnels after crawling through dozens of them when her curiosity got the better of her.

“I just love finding history that people don’t really know about,” she said.

Many of the tunnels were built to connect restaurants, hotels and shops throughout downtown.

“Grocery stores, butcher shops, laundry. You had some kitchens for different restaurants. They would possibly cook it one place and then would go through the tunnels to deliver it to different hotels,” she said.

Some were also built in the wake of race riots in the late 1800s, when Denver’s Chinatown was destroyed, as a place for those workers to hide.

The tunnels were naturally located underneath some of the city’s oldest buildings, including Union Station and the Oxford Hotel. Others existed beneath what was then Denver’s "red light district" on Market Street.

“They would come through the tunnels and up into the buildings, and then sneak up the stairs to where the girls were,” Beach said.

Denver’s most elaborate building also had a tunnel system. Tunnels beneath the State Capitol connected nine buildings throughout the Capitol complex and run for entire blocks under city streets and sidewalks.

Doug Platt is the spokesman for the state Department of Personnel and Administration. He has walked every inch of those tunnels. While there are many rumors about why they were built, they actually served a very practical purpose.

“There’s actually ore car tracks you can still see today. ... When they poured the floor, they left those tracks visible. Back when they built the Capitol, this building was heated with coal,” Platt said.

Mules and coal dust weren’t the only subterrain hazard workers had to watch out for. A river of sorts also once ran underground in the form of open sewage.

“Sewer from the building would drain down from gravity and then run through what at the time were open trenches,” Platt said.

However, it wasn’t all muck and mules. The elaborate maze of tunnels beneath the Capitol also once hid treasure locked away in giant vaults.

“A hundred years ago, the state actually stored its treasury down here. That was the days before ATMs,” Platt said.

Rare pieces of marble are also still securely stored underground. Rose onyx is used on the main floors of the Capitol. The same can be said for Denver’s tunnel system.

The elaborate network once connecting dozens of buildings is on the verge of extinction. Most have been destroyed because of safety concerns.

“They really need to save these things. You can’t just keep saving trolley tracks and fancy dresses. This is also part of our history,” Beach said.

“It's really a window into a century and a lifestyle that's long gone, and it's really a pleasure to show it to the people of Colorado,” Platt said.

It’s hidden history right beneath our feet.

Beach's book, "The Tunnels under Our Feet,” is about the 14 Colorado towns with an underground tunnel network.