If you've ever studied the skyline of Paris, you might have noticed that the central part of the city has virtually no tall buildings. Seems odd for a bustling European metropolis, right?
Well, that's because nobody can build much of anything in Paris without the city collapsing into the earth, thanks to a chaotic maze of unmapped tunnels dug underneath it.
Gypsum and limestone had been mined beneath Paris since the 13th century. As the city grew, so did the tunnels, but nobody bothered to keep track of how many were being dug or how far they extended in any particular direction.