DENVER (AP) — A fire raging in an underground Colorado coal field in 1883 sent so much smoke pouring from cracks in the ground that the scene was likened to burning volcanoes and the state’s first mining inspector deemed the blaze “impossible to extinguish.”

Nearly 140 years later two fires still smolder in the now-abandoned coal field near Boulder — the same area where a wildfire last month destroyed more than 1,000 homes and buildings and killed at least one person.

It’s still unknown what caused the December blaze that became the most destructive in Colorado history, but Boulder County authorities have said they’re investigating the area’s abandoned coal mines as one of several possible causes, along with power lines, human activity and other possibilities.

Could smoldering coal have started such a fire? History shows the answer is yes, with at least two Colorado blazes in the past 20 years blamed on mine fires that spread to the surface. And in Montana this past summer slow-burning coal reserves fanned by winds sparked a pair of blazes that burned a combined 267 square miles on and around the Northern Cheyenne Indian Reservation.

Across the U.S. at least 259 underground mine fires burned in more than a dozen states as of last September, according to federal Office of Surface Mining data. There are hundreds and possibly thousands more undocumented blazes burning in coal seams that have never been mined, researchers and government officials say.

Globally, such fires are also a problem, including in India, Australia and South Africa. In China, the world’s largest coal producer, an estimated 10 million to 200 million tons of the fuel annually burn or are left inaccessible by fires, according to the U.S. Geological Survey.

As climate change leads to drought across larges swaths of a U.S. West already seeing longer and more destructive fire seasons, experts say smoldering coal fires will pose a continuing threat.

Such fires can be ignited by lightning, humans and even spontaneously at temperatures as low as 86 degrees Fahrenheit, said Jurgen Brune a Colorado School of Mines engineering professor. Many are impossible to put out, slowly burning underground as the combustion feeds off a small amount of oxygen present in the coal, he said.

“Covering it up and trying to take away the oxygen from the fire puts out most fires. Not for coal fires,” Brune said.

Underground coal seams burn unpredictably and can break through to the surface without warning long after a fire starts, he said.

“It’s like trying to predict an earthquake,” Brune said. “With all the technology we have today they are not coming any closer to predicting them. The same goes for a coal fire.”

The fires emit toxic mercury and the greenhouse gas carbon dioxide, and cause sinkholes when the ground’s surface collapses into burned cavities below. In Centralia, Pennsylvania, the fumes and subsidence from a coal fire that started beneath the town in 1962 got so bad that more than 1,000 people eventually relocated at a cost of $42 million.

The estimated future cost to control the 200 known abandoned mine blazes across the U.S. is almost $900 million, according to the Office of Surface Mining database.

In the wake of last summer’s fires, local officials in Montana plan to map out burning seams across the state’s southeast in coming months using a federal grant. Controlling them will be difficult and could cost a minimum of $300,000 per site, said Bobbi Vannattan with the Rosebud Conservation District, which is helping to coordinate the mapping.

“The problem with coal seam fires is we don’t know how deep they are or how wide they are until you get in there and start digging,” she said.

In Colorado, officials were monitoring at least 38 underground coal fires as of 2019.

Boulder County Sheriff’s Office spokesperson Carrie Haverfield declined to specify which mines were being investigated in relation to the Dec. 30 blaze or what prompted authorities’ interest, which was first reported by KUSA-TV.

At least three efforts were made by authorities to quench or reduce damage from the blaze the state mine inspector first encountered more than a century ago at the abandoned Marshall Mine, located on park land in the vicinity of where investigators believe the recent fire started.

The first came in 1982, when federal officials drilled into the ground to investigate how far the fire extended and later sought to smother it with a 2-foot layer of dirt, according to a U.S. Geological Survey report.

In 2005, a small brush fire was traced back to a hot vent from the fire. Crews from Boulder removed vegetation in the area and federal officials tried to fill the vent and others with rocks to try to protect against another fire, according to the Office of Surface Mining. In 2016, after federal officials said the state had taken control of the site, workers excavated and filled in two areas where ground had subsided after fire consumed the underground coal.

When the site was visited two years later by workers for a state contractor, Tetra Tech, they found no evidence of fire — no melting snow, no smoky odors. The company recommended annual monitoring of the site because of its past erratic behavior but no new steps to abate the potential hazard.

“The fire’s activity is very low and thus presents little potential to start a surface fire,” Tetra Tech wrote in a 2019 report prepared for the Colorado Division of Reclamation, Mining and Safety.

Generally the responsibility of monitoring coal mine fires falls to the property owner, according to the Colorado Division of Reclamation, Mining and Safety. The Marshall Mine fire sits under an open space park owned by the city of Boulder near a state highway.

Boulder officials said they don’t have responsibility to do work on the fire. However, the city is required to notify the state if rangers or trail crews at the popular hiking area see smoke or shifts, city spokesperson Sarah Huntley said.

The state’s mining division said it was not notified of any changes at the mine fire since the 2019 report.

The state receives federal funding to help property owners monitor or mitigate abandoned coal mine fires. But the mining division cannot require any work be done at the sites, spokesperson Chris Arend said.