When you walk down a sidewalk in, say, downtown Denver or Colorado Springs, there just might be tunnels underneath your feet. They’re known as “hollow sidewalks,” and many of them have been filled in or sealed off over the years. But plenty of them are still there.

Cañon City author Tracy Beach has spent years looking for these subterranean passageways around Colorado. Her new book is called, “The Tunnels Under Our Feet: Colorado’s Forgotten Hollow Sidewalks.”

She spent five years documenting them. Most are connected to buildings and she had to convince owners to let her explore with flashlight and camera. Some had no idea what the tunnels were originally used for. Some allow for delivery of products from the street level through openings in the sidewalks. Others, such as tunnels connecting Denver’s Union Station to nearby hotels, were used as corridors.