They have a pretty long story but I am going to try and summarise the main and interesting points, they are an underground ossuary located to the south of the former city gate. The catacombs hold around 6 million peoples remains and are the old stone mines, they have had to be monitored due to prior vandalism.
Like many cities that had their Christian dead buried in the consecrated city grounds Paris suffered overcrowding. The dead who were poor were mass buried to ensure that they tried to ease it but by the 17th Century it was all too much and they needed a new method.
New cemeteries were built, larger scaled ones that could accommodate more dead and the abandoned stone mines were chosen so that they could arrange to exhume the dead, and transfer them into the newly appointed catacombs. To begin with the catacombs were just a bone depository but eventually the stones, tomb and bones were arranged into decorations.
During August 1788 the riots from three areas meant that more bodies were brought into the catacombs. Val-de-gráce’s doorkeeper, Phillibert Aspairt, was lost in the catacombs in 1793, his body was found 11 years later and his tomb is at the spot where his body was found. Some of the graffiti there dates back to the 18th Century, Parisian members of the French Revolution also used them too.
Unfortunately it’s not all good, due to the catacombs being under the Paris streets it means that they cannot build any large foundations. And of course the catacombs are a hot-spot of hauntings.