The phrase 'eternal flame' brings up thoughts of magic, of wizards and witches (or scientists) messing with natural laws, and summoning things from hell*.
Or it conjures up vomit worthy, ultra cheesy, soft-rock lyrics.
But the point is that eternal flames do actually exist: They happen where a natural gas reserve breaches the surface, and gets ignited by some natural event.
To give you an idea of how tenacious these natural fires can be, here's one that burns underneath a waterfall:
That one is in Erie county, New York. it would have seemed damn spooky to many people, even just a hundred years ago - here's a link to a paper written about it by the Indiana Geological Survey. But today it just seems to underline a point that the world keeps making: No matter how smart humankind thinks it is, nature has usually got there first....