Subterrene

Then he saw a sort of grey phosphorescence about, and guessed they were coming even to that inner world of subterrene horror of which dim legends tell, and which is litten only by the pale death-fire wherewith reeks the ghoulish air and the primal mists of the pits at earth’s core.

With a deafening screech of metal upon rock—which surely must echo through all the recesses of the Mountain, and waken all its nightmare brood!—the subterrene smashed through the wall and came to rest beside them.