An architect I knew in Paris once pointed out to me that everything that the entire built environment in Paris was quarried from the limestone beneath it; there's as much of the city below ground as there is above it.
Architect from Paris here.
That’s right, except the mines didn’t cover all of Paris, they were originally outside and eventually built over. [1]
The underground networks below Paris are actually said to be even more extensive than the streets and extend many stories deep. These networks are the result of a complex layering of quarries, catacombs, sewers, bunkers, undergound metro, and even waterways [2], large parts of which have been abandoned over time.
No public map is available, who knows what else might be hiding there.
