http://www.catacombes-de-paris.fr/english.htm

As Paris grew and became more densely populated, the city needed both the space, and a way to separate the remains of those who had died from many diseases from the living, in an attempt to keep the population at least somewhat more healthy.

As a result, the contents of the St. Eustache cemetery, which had been used for hundreds of years, was closed in 1785, and the remains moved to the Catacombs, which are still (mostly) open to visitors today. (other cemeteries around the city were added later, as were the remains of those executed in the Revolution)

I know they painstakingly listed all known burials in a register, but I'm not sure how to access that information.