https://en.wikipedia.org/wiki/Burning_Mountain
A long list of similar fires:
https://en.wikipedia.org/wiki/Coal-seam_fire
In Colorado, coal fires have arisen as a consequence of fluctuations in the groundwater level, which can increase the temperature of the coal up to 300 °C, enough to cause it to spontaneously ignite.
Here it is, almost a 100 years ago:
https://localhistory.boulderlibrary.org/islandora/object/isl...
"Across the world, thousands of underground coal fires are burning at any given moment. The problem is most acute in industrializing, coal-rich nations such as China.[7] Global coal fire emissions are estimated to cause 40 tons of mercury to enter the atmosphere annually, and to represent three percent of the world's annual CO2 emissions.[8]"
From the wikipedia article above