The catacombs came to be because the limestone had been dug out of the undergrounds of Paris to build cathedrals and such. The tunnels were left behind and inspected with this alpha numeric system. The initials of the inspector and the year
The bones were moved from many cemetaries all over the city that had been sent into disrepair. the bones could be relocated into this place that was otherwise just empty tunnels. This is the largest burial ground now
The stone could be mined quite far up and then it needed to be reinforced and form this bell shape up in the air.