The eternal flame
Coal is the answer, as Hank Green explains below. It's nothing more than flammable rocks, and when it gets going underground, it can be impossible to put out, as the town of Centralia discovered when a coal seam started burning back in 1962.
There is enough coal in the Centralia seam to keep the fire going for another 250 years, but that's nothing compared to Australia's Burning Mountain, which has an underground coal fire that has been burning for the last 6,000 years. Check it out. --Ryan Cooper