A lot of the mass graves have since been built over--I believe the one that Robespierre was probably buried in had a dance hall built on top of it and then a public swimming pool. Surprisingly, though, I haven't read too much about remains being accidentally exhumed during modern construction projects. Does anyone know anything on this?

As for the catacombs, if memory serves me, some of the bodies are there. I know that they were used to bury victims of a cholera outbreak earlier that century and that, during the Terror, they were relatively popular places to hide if you found yourself labeled an enemy of the republic.