Discovery of historic tunnels and sewer system under 16th Street in Denver cost the city $18 million

If you’ve walked down 16th Street in the last few years and thought, “Wow, this construction is taking forever,” there’s actually a wild reason for that. Workers renovating Denver’s iconic pedestrian promenade discovered a historic brick sewer system buried underneath the street, tunnels that nobody planned for, nobody budgeted for and nobody apparently knew were down there until a construction crew dug into them.

According to a Denver Auditor’s report released this month, the discovery of the historic brick sewer added $18 million in cost overruns to the 16th Street Improvement Project and caused a nearly year-long delay. The City Council originally approved $149 million in renovation costs back in 2021, and the brick sewer was discovered the following year, halting construction while the city figured out what to do with tunnels that had been sitting under one of Denver’s busiest streets for well over a century. Denver Auditor Timothy O’Brien didn’t mince words: “Because the city did not consider the sewer while planning the 16th Street Improvement Project, businesses on 16th Street had construction lingering outside their doors for nearly a year longer than expected.” The total project cost ballooned to $175.6 million. One of the three qualified contractors actually withdrew from bidding because of the utility risks.

The tunnels themselves are a fascinating piece of Denver history. The city has a well-documented network of underground passages dating back to the late 1800s, originally built for coal delivery, connecting basements of downtown buildings so goods could move beneath the streets without blocking traffic above. During Prohibition, many of those same tunnels were repurposed for bootlegging, with speakeasies featuring trapdoors and hidden stairways that led directly into the underground network. Some passages reportedly connected Union Station to the Oxford Hotel and the Colorado Capitol building to multiple downtown locations.

So the next time you’re strolling 16th Street and wondering why the renovation took so long, just know: it’s because Denver literally had secrets buried under the sidewalk. And those secrets cost us $18 million.