Throughout the fall and winter months, an underground fire sent billowing smoke out of a fissure in the mountainside above Kenilworth, Utah. The fire, burning in a coal seam in the historic Kenilworth mine, may have been smoldering for a century, or it may have been the product of spontaneous combustion.

"Coal can ignite at pretty low temperatures," said Steve Fluke, Program Manager for the Abandoned Mine Reclamation Program, part of Utah's Department of Natural Resources Division of Oil, Gas & Mining (DOGM). "You can put the fire out, but the rock around it can be hot and reignite it, or the coal itself can be smoldering for a long time given the right conditions."

The Kenilworth fire was first reported in 2021 and had been smoldering with very little surface expression, so the DOGM didn't see a need to address the problem at that time. But Kenilworth citizens alerted the agency when heavy smoke was seen coming from a new opening in the mountain, a cleft caused by a cave-in of the underground tunnels.

Kenilworth—population 95—sits high in the Bookcliff Range above Helper. Like many Carbon County towns, it was once a coal-mining and railroad camp. Today, photos and artifacts from the Kenilworth mine can be seen at the Helper Museum, but the mine has been closed since 1968.

In 1961, the Kenilworth mine was merged with the infamous Castle Gate mine by new owner Independent Coal and Coke, creating one mine 10 miles long and with 30 total miles of tunnels. Castle Gate is known for one of the country's biggest mining disasters—a 1924 methane gas explosion which claimed the lives of 173 married men, who left behind their wives and 417 children.

Another explosion in 2000, when the mine was named Willow Creek Coal, took the lives of two miners and closed the mine for good. But since August, Kenilworth residents have been dealing with heavy, dark smoke coming out of the mountain.

"Coal within the Castle Gate and Kenilworth mines burns easily because it's so pure," said Helper Museum director Roman Vega, referring to Carbon County's widely-regarded "clean" burning coal. "The black smoke comes from the wooden timbers and railroad ties burning."

The Helper Museum provided maps of the mines to project planners. The historic maps showed the underground workings in great detail, allowing planners to see where coal seams connect and where the original ventilation shafts were placed. They guessed that the ventilation shafts were feeding the fire.

In the early stages of the project, Fluke said the planners had identified five sources of ventilation. The first course of action was to seal those so they stopped drawing so much air.

"The idea is that plugging up these portals and various openings in the mine will have an effect like closing the flue on a fireplace," Fluke said.

Vega suggested that the Castle Gate Mine's No. 2 Portal, which had been sealed with cinder blocks, concrete, and dirt after the 1924 disaster, was providing the major source of air. His proposition was supported by other locals.

"If you go up there, you can just hear it, you can feel the air pulling through it," confirmed Dean Marchello, President of the Carbon County Historical Society.

Marchello worked at the Carbon Power Plant in Castle Gate for 31 years.

Crews sealed the No. 2 Portal with expanding foam during Thanksgiving week, which reduced the volume of smoke at the main vent above Kenilworth. They continued sealing seven additional vents and openings, and by Dec. 12, the smoke had decreased by two-thirds.

Jan Morse, an environmental scientist and spokesperson for DOGM announced in a public statement that because of the reduction in smoke volume, crews could safely access the west bench to seal another vent spewing smoke above Kenilworth.

During the week of December 8, crews successfully sealed this vent, which largely eliminated the smoke impacting the town and its residents. However, while the crews were conducting work on the portals located to the east of the main vent, they discovered heat at the portal seals, indicating the area where the fire continues to burn underground.

The DOGM suggested that residents avoid the area and stated that they plan to investigate the site further in the spring.

Funding for the project comes from the Office of Surface Mining Reclamation and Enforcement, a federal agency that regulates coal mining and ensures reclamation of the land around closed mines.

The agency collects a fee on coal production which is saved in a trust for situations like these.

There are currently seven other underground fires burning in Utah. It's not necessary to put them out, experts say, because they're not associated with abandoned mines.

"These natural fires can only go so far into the mountain until they don't have any more air," Fluke explained. "But with a coal mine, they have a pretty much endless supply of oxygen. So they are more prone to collapsing and can go deeper into the mine workings."