The bible discibes it as the area of slim pits i.e. asphalt.
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.