DENVER (KDVR) — Work is set to begin after increased activity was noticed at an underground coal mine fire that’s been burning for more than 50 years near where the Marshall Fire started.

According to the Colorado Division of Reclamation, Mining and Safety, the Lewis Coal Mine Fire has shown increased activity since late 2021 when the Marshall Fire tore through parts of Superior, Louisville and unincorporated Boulder County.

Two sites have been identified for work: the Lewis site, which sits north of Marshall Road and east of Cherryvale Road, and the Marshall site, which is at the Marshall Mesa Trailhead.

According to DRMS, the fire is at risk of causing the Davidson Ditch, a water supply ditch, to collapse when it fills in the spring.

Jeff Graves, director of the Inactive Mine Reclamation Program for DRMS, said this is, “a proactive effort to reduce the potential for surface ignition as a result of the increased underground coal mine fire activity.”

Since the Marshall Fire, observers have reported more activity at the Lewis site “including increased surface subsidence, surface fracturing and cracking, and higher surface temperatures.”

A report from DRMS said the rainy weather this year has added to the severity of these conditions.

Work at the Lewis site is set to begin in October and be completed by March 1, 2024. Work for the Marshall site is set to take place following that completion date.

The work will include excavation of the sites below the mine and the removal of burning or smoldering coal. That coal will then be mixed with rocks and soil and returned to where it came from.

An FAQ shared by the DRMS explains how the site will be monitored and what will be done in the event of strong winds or a red flag warning.

An online information session on the upcoming mitigation work at the Lewis and Marshall sites will take place on Aug. 28 at 5:30 p.m. on Zoom.

Investigators said the cause of the Marshall Fire was a combination of embers from an extinguished fire and a damaged Xcel power line, but there was speculation during the investigation that the underground fire may have contributed. Investigators said this was unlikely but could not be ruled out.

There are approximately 1,736 known abandoned coal mines in the state, according to the Colorado Underground Coal Mine Fires 2018 Inventory Report.