One of the wildfires burning in Colorado was started by flames from an underground coal fire that "may have been burning since 1910." How can a fire burn underground for 92 years, and why hasn't anyone put it out before now?
Underground fires usually begin when a coal seam juts up through the ground's surface. The coal can be ignited in three ways: by human accident, by lightning, or by spontaneous combustion—the process by which the explosive gases contained in coal combine with oxygen and heat up to the point where they burst into flame. (This process typically starts underground where the heat can't be dissipated into air.) When the seam ignites, the flames spread to burn the adjacent, underground coal.
But fire needs oxygen to burn. So what keeps underground fires burning for decades? Once a portion of the coal has burned, it turns to ash. Since the ash can't support the weight of rock layers above, the layers buckle, creating cracks and crevices where oxygen can get through and rejuvenate the fire. Underground fires are also sustained by mineshafts, which provide a steady stream of oxygen to the inferno.
Why don't forest rangers just put out underground fires? First, they have to find them. Underground fires are hard to pinpoint from the surface, and rangers often need satellite equipment (not yet widely available) to properly do the job. If they manage to find the fire, a special foam called cellular grout can be pumped into the ground to fill up holes and keep oxygen out. Unfortunately, cellular grout is prohibitively expensive, and it's difficult to plug all the leaks. Even good old-fashioned water is useless. Coal fires can reach temperatures of 1200 degrees Fahrenheit, so water dumped on them evaporates instead of putting them out.
Explainer thanks David Noe of the Colorado Geological Survey and Glenn Stracher of EastGeorgiaCollege.