Not too far underneath the soil. Tar/bitumen/oily type stuff.
Some have hypothesized that explosions actually started from the ground, and launched all the fiery stuff into the air, and then it all came back down on them. Would make sense as there was probably more than enough natural gas in all that stuff to be used as projectile fuel.
Except that isn’t what the Book says. Since the Book says that God rained fire and brimstone from the sky; that’s what STARTED it. The fired from heaven would then have ignited the petroleum products.
For another instance of fire coming out of the sky, check out Elijah’s showdown on Mt. Carmel.