The Mine

"The coal mine was chock-full of bodies. Blue faces, eyes screwed shut and covered in ash, seemed twisted to inhale the air of death lingering in the tunnels. Bloated fingers clawed the darkness, reaching up for anything. The entire place had collapsed in on itself from the sheer amount of tunneling. With no trees to make supports and the steel mills in ruin, the passages buckled from the earth’s weight above, crushing every living thing below. The ventilation shafts were opened and soldiers poured liquid concrete down the long pipes, a slurry of chemicals and sand. The oozing mixture had covered every body, dead or alive, encapsulating the workers forever. The concrete would keep a collapse from happening again, and the veins of liquid rock hardened and held the soil together. Collapses certainly needed to be prevented, for up above there was activity that couldn’t be stopped."

"The Mine" is the collapsed underground coal mine mentioned in Crimson Earth: Sodom and Gomorrah.

Location
The coal mine is directly below Gomorrah, a large camp near sister camps Sodom and Camp 19. The underground mine is apparently a gravesite, with countless corpses entombed in solidified concrete. The mine's solid concrete state is presumably what is keeping Gomorrah above ground, as the foundation is torn out from under it.

Role in the series
The primary author acknowledges the coal mine will be a pivotal location in upcoming chapters. To what extent it will be pivotal is left to debate.