I recently turned in a short story which featured a natural gas fire burning eternally in deep in the forest. This was based on real life sites, but when I started reading up on this rare phenomenon, it was slightly less rare than I’d thought!
Here’s a quick list of fascinating sites which might be on fire at any given moment: https://www.treehugger.com/naturally-occurring-eternal-flames-4868390
In my story, such fires became religious sites, where the eternal flame helped to represent another infinite light. But it’s pretty cool to observe these phenomena on any level. Maybe someday I’ll visit such a place!