This truly is the stuff of nightmares. When I was living in Paris I got to know the staff of a dive bar in the Mouffetard. Late one night the bartender kicked everyone out and he and I had a couple of drinks. He asked me if I wanted to see something cool, so of course the answer was yes.
He took me downstairs into their liquor storage room and moved a few boxes, revealing a trap door into the catacombs. I climbed down into the narrow tunnel and he closed the trap door above me, and I don’t think I’ve ever seen darkness like that before or since. Moving ten minutes in any direction would have had me lost for all eternity.