“The practice common then for burying the lesser-wealthy dead was mass inhumation. Once an excavation in one section of the cemetery was full, it would be covered over and another opened. “Few of the dead buried in this way had the privilege of coffins; often the casket used for a burial ceremony would be re-used for the next. Thus the residues resulting from the decaying of organic matter, a process often chemically accelerated with the use of lime, entered directly into the earth, creating a situation quite unacceptable for a city whose then principal source of liquid sustenance was well water.”
http://en.wikipedia.org/wiki/Catacombs_of_Paris