Most of all of Paris' larger churches once had their own cemeteries, but city growth and generations of dead began to overwhelm them. From the late seventeenth century, Paris' largest Les Innocents cemetery was saturated to a point where their neighbours were suffering from disease, due to contamination caused by improper burials, open mass graves, and earth charged with decomposing organic matter. Other cemeteries also started to experience the same issue.To solve the problem, the government decided to discreetly move millions of remains in the overflowing cemeteries to the Catacombs of Paris, an underground maze of abandoned tunnels and chambers which created one of the world's largest public mass graves. *
I also found an interesting video about a man getting lost in the Catacombs. I believe the story is fictionalized, but it's still worth checking out just for the footage of the graves:You can see Part#2 of the video here.