The catacombs of Paris originally had nothing to do with death. They began as a network of quarries beneath the city, providing gypsum to build the metropolis. After they'd been mined, the tunnels stood empty and unused.
Simultaneous with the reconstruction of Paris in the 1780s, a movement gained momentum to clean out the old churchyards. The most notorious of them, the Cimetière des Innocents, had been in use since the Middle Ages. Accounts of the period speak of a pestilential hellhole, jammed with liquefying cadavers. Fearing epidemics, the city fathers overrode the protests of the clergy.
In 1786, after the ossuary was filled, the Archbishop of Paris consecrated the bones of approximately 6 million people.