So this happened a couple weeks ago, but with school starting up life has been pretty crazy here in Paris! One of the things I was most looking forward to seeing while I was here ware the catacombs - aka 6 million skeletons stacked in old quarries underneath the city.
Long story short, the quarries had been dug for centuries to get limestone out of the ground for construction of parisian buildings. Those which were exhausted of their minerals were basically forgotten about until they started crumbling down and either had to be filled in or better supported.
Around this time, (late 18th century) cemeteries in the city began getting too full. This hadn't been an issue before because predeceasing the rise of Christianity in the country, people were buried outside of the city. However, hundreds of years after they had to be buried in sacred soil, there wasn't enough room for them anymore, considering Paris was already overpopulated by the living. And these full graveyards started becoming serious health hazards for the city. Transferring all of Paris's dead to the tunnels took from 1786-1788.
From 1788-1810 the tunnels were merely bone repositories, until a renovation rearranged the skulls & femurs into their current configuration and used tombstones as decorations.
So yeah, check it out! Also, check out the bar 'katacombes' in Montreal modeled after the parisian catacombs.