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.

[1] https://en.wikipedia.org/wiki/Mines_of_Paris [2]https://commons.wikimedia.org/wiki/File:Le_Canal_Saint-Marti...