Another possibility:
Deadly Colorado blaze renews focus on underground coal fires
By MATTHEW BROWN and COLLEEN SLEVINyesterday
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 (691 square kilometers)
on and around the Northern Cheyenne Indian Reservation.