I am a DM for a homebrew D&D 5e game. In terms of technology, the time period loosely takes place somewhere between 1100 and 1400 CE (so High Middle Ages to Late Middle Ages).
The villain of my piece knows of some substance (TBD) underground that, when ignited, would destroy everything within several miles of the ignition point. The problem I am running in to is I would have no idea what kind of substance that would be.
I was thinking it could be some kind of naturally occurring gas, but I am not sure if something like that exists, or exists but is not concentrated enough to be able to cause the amount of damage I would need it to. I should also mention that the substance needs to be something that has existed and/or still exists in the real world. I don't want magic to have anything to do with the villain's scheme.
Does anyone know of a substance that could fulfill the requirements that I need it to?