Eternal Flame

 A burning mine

New Straitsville was originally founded in 1870 as a coal mining town by the New Straitsville Mining Company. The town grew quickly and by 1880 the population was over 4000 people. The coal mining activity ended in 1884, when a labor dispute at the mine ended with a group of miners sending a burning coal car into the mine, igniting the coal. At one time the heat from the fire was so great that residents could draw hot water directly from wells to brew coffee. The fire in the New Straitsville mine burns to this day.

I thought this was an isolated thing.  I was wrong

Many coalfields in the USA are subject to spontaneous ignition. The federal Office of Surface Mining (OSM) maintains a database (AMLIS), which in 1999 listed 150 fire zones. In mid-2010, according to OSM, more than 100 fires were burning beneath nine states, most of them in Colorado, Kentucky, Pennsylvania, Utah and West Virginia. But geologists say many fires go unreported, so that the actual number of them is nearer to 200, across 21 states.

I've based entire roleplaying game campaigns on Centralia Pennsylvania with the underground mine being a gate to hell that the players have to enter and/or destroy.

but there are bigger ones