I was always wondering what Drows used to set non-extinguishable fire on cobblestones: thermite, or napalm. I know it can't be the former, as thermite burns quickly, and stone can't be ignited by itself. Apparently, it was napalm.
I expect it's Geek Fire, just look at various flame wars that Geeks will wage that never seem to go out!
Wait, we just came from the Nine Hells, it must be Hellfire! The secret is only Holy Water will put it out!