Way back when, Paris had mines under the city. At some point (in the 1800’s, I think) the abandoned mines started to collapse and cause problems for the city. The city began a project to fix this, to reinforce and modernize the mines. Around the same time, the city also started running out of cemetery space, and somebody had the macabre/genius idea of using skulls and femurs to decorate the newly reinforced Catacombs
TBH, I wasn’t listening that closely to the self-guided audio tour, I just wanted to see the tunnels with bones.