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Danger from underground coal fire is well-known, but did it play a role in Marshall Fire?

Rangers documented multiple instances in which the subsurface fire raised concerns, according to a 9Wants to Know examination of hundreds of pages of documents.

BOULDER, Colo. — When the City of Boulder began buying up land in the 1970s in an area of long-abandoned coal mines, everyone acknowledged that below the surface, a fire had been slowly burning for a century.

That fire, in the web of coal seams that underlay the area near what is now the Marshall Mesa trailhead, is still burning.

While generations of people were used to the occasional sight of smoke and steam rising from the ground, there’s a very real possibility that the coal can burn through the surface.

That was what happened in December 2005. Park rangers have documented other instances in which the subsurface fire raised concerns, according to a 9Wants to Know examination of hundreds of pages of documents that cover Boulder’s ownership of the land, which was acquired for open space.

RELATED: Officials warned Boulder after 2005 wildfire that underground mine fire was 'imminent hazard,' long-term danger

Now, with investigators trying to determine whether the burning coal played a role in the devastating Marshall Fire – which leveled more than 1,000 homes and killed two people – there’s new focus on the potential danger.

“We've been aware of the historical context, as well as potential risks associated with the former coal mine in that location, for years,” said Sarah Huntley, a spokeswoman for the City of Boulder. “And we were led to believe that the activity underneath was of limited risk, certainly some risk, and that that was going to be a manageable risk.”

Burning for 150 years

Even before Colorado was a state, coal mining companies turned their attention to the area south of present-day Boulder, where rich veins of coal lay beneath the surface.

By 1869, some of that coal was burning. The cause isn’t known, but blasting, a cooking fire and even an illicit still have been listed as potential causes.

That fire kept burning, even after the mines in the area were abandoned in the 1930s and 1940s.

By the 1970s, a sustained effort to build an open space system for the enjoyment of area residents was underway, and the city acquired multiple parcels over abandoned coal mines.

Even in those early years, there was controversy over what should be done about the underground fire.

In 1975, the former operator of one of the mines accused Boulder of breaking a promise to extinguish the blaze, according to an article published May 29, 1975, by the Fort Collins Coloradoan.

Credit: Courtesy City of Boulder
The City of Boulder has posted signs like this in the past to warn open space visitors of the potential danger posed by an underground coal fire.

What was actually agreed to wasn't clear. When 9Wants to Know requested a copy of the purchase agreement, the city could locate only a single document. It contains no mention of the coal fire.

“Other than your citation of this newspaper article, we have no knowledge of any specific steps that we pledged to take or committed to taking to minimize the risk of the fire,” Huntley said. “That being said, we do feel like the records show that we have taken significant action over the years and that we’ll continue to do so as the need arises.”

Some of that action was the result of a December 2005 wildfire sparked when burning coal broke through the surface.

Weeks later, the state brought in 275 tons of gravel that was used to fill cracks in the surface and cover the area. In 2016, more ground work was done.

RELATED: Underground coal fire being investigated as potential Marshall Fire ignition source

Rangers have noted other instances in which the underground fire was visible from the surface, including a time in December 2012 when smoke rose from a hole in the ground not far from the trailhead.

Huntley said the city has been “very vigilant” in monitoring the potential danger.

“We estimate that because of the kind of work that goes on out there – which includes trailhead maintenance, trail monitoring, vegetation monitoring, habitat restoration, educational programs, ranger patrols, and we also have some agricultural grazing in that area – that we probably have on average ten to twelve staff members out on that location on a weekly basis,” Huntley said.

'Virtually impossible' to extinguish

Credit: Courtesy City of Boulder
This photo from the 1960s shows burning underground coal visible from a slot in the ground in an area that is now Boulder open space.

Underground coal fires are a fact of life in parts of the country. There are an estimated 38 burning in Colorado, according to the latest state report – and they are all but impossible to extinguish.

“One of the things that coal seam experts tell us is that it is virtually impossible, if not impossible, to completely extinguish a fire underneath the surface,” Huntley said.

A coal fire that began in the 1960s in Pennsylvania led to the abandonment of an entire town and the relocation of a highway. According to some estimates, that fire is expected to burn for 250 years.

Now the question for Colorado investigators is whether the fire burning beneath open space played a role in the start – or the spread – of the Marshall fire.

RELATED: Videos show Marshall Fire started by 2 separate ignition points less than a mile apart

“We are obviously very, very concerned about any possibility that our property might have played a kind of contributing role to this terrible event,” Huntley said.

Eight days after the Marshall Fire, a drone fitted with a thermal camera was flown over the area. It identified abnormally high temperatures: 277 degrees in one area, more than 300 in another.

What was not clear was whether the existing coal fire burned to the surface in those spots or whether the Marshall Fire ignited coal near the surface.

As the investigation continues, extensive drilling work is underway in an effort to get an accurate picture of the extent and location of the underground fire.

Contact 9Wants to Know investigator Kevin Vaughan with tips about this or any story: kevin.vaughan@9news.com or 303-871-1862.

RELATED: Wildfire expert: Marshall Fire may have started in multiple places as a result of multiple ignition points

RELATED: Group drops effort to require Boulder County to seek new cleanup contract

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