side passage
subterannean cavern
spiral stair to crypts

"After more aeons of descent I saw some side passages or burrows leading from unknown recesses of blackness to this shaft of nighted mystery. Soon they became excessively numerous, like impious catacombs of nameless menace; and their pungent odour of decay grew quite unbearable. I knew we must have passed down through the mountain and beneath the earth of Kingsport itself, and I shivered that a town should be so aged and maggoty with subterraneous evil. " -- The Festival