We made it in on the 2nd day attempt. (There is always a long line-up so make sure you give yourself extra 30mins to a 1hr for lining up.) Once you make it through the entrance (€8), it is a self-guided tour with or without an audio guide(€4). We opted for an audio guide and appreciated the explanation. Sometimes, there are descriptions on the wall but in the 19m deep into the darkness, I recommend the audio guide. It is worth the 4euros. Self-guided also meant there is a plenty of time to take pictures and walk through at your own pace. The display of skulls and bones are in multiple quarries so don’t get queued up on first quarry that you see..there are a lot more to see!
The Catacombs contain the remainders of approximately six million Parisian! Opened in the late 18th century, the underground cemetery became a tourist attraction on a small scale from the early 19th century, and has been open to the public on a regular basis from 1874.
Since Roman times, Paris has buried its dead on the outskirts of the city, but habits changed with the rise of Christianity and its practice of burying its faithful in the consecrated ground under and around its churches, no matter their location. By the 10th century, many of Paris’s parish cemeteries were well
within city limits, and eventually some, because of their central location in dense urban growth, were unable to expand and became overcrowded. An attempt to remedy this situation came in the early 12th century with the opening of a central mass burial ground for those not wealthy enough to pay for a church burial. Once an excavation in one section of the cemetery was full, it would be covered over and another opened. 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 unacceptable for a city whose then-principal source of water was wells.
The government had been searching for and consolidating long abandoned stone quarries in and around the capital since 1777, and it was the Police Lieutenant General overseeing the renovations, Alexandre Lenoir, who first had the idea to use empty underground tunnels on the outskirts of the capital to this end. His successor, Thiroux de Crosne, chose a place to the south of Paris’s “porte d’Enfer” city gate (the place Denfert-Rochereau today), and the exhumation and transfer of all Paris’s dead to the underground sepulture began in 1786, taking until 1788 to complete.
The catacombs in their first years were mainly a bone repository but in 1810, Louis-Étienne Héricart de Thury, oversaw the renovations that would transform the underground caverns into a real and visitable sepulture on par with any mausoleum. In addition to directing the arrangement of skulls and femurs into the configuration seen in the catacombs today, he used those tombstones and cemetery decorations he could find (many had disappeared after the 1789 Revolution) to complement the walls of bones. – Wiki
One of those must-see places if you are in Paris! :)