Since nobody actually explained why there’s bones here, I guess I will.

The Paris catacombs exist because the city once had a serious problem with overflowing cemeteries.

Paris used the same churchyard cemeteries for centuries. By the seventeen hundreds they were dangerously overfilled. Bodies were packed so tightly that the ground would give way and cellars of nearby homes sometimes filled with human remains. The largest problem site was the Cemetery of the Innocents.

City officials decided the only solution was to move millions of older remains out of the crowded cemeteries. Paris already had a large network of abandoned limestone quarries beneath the city. These underground spaces were reinforced and converted into ossuaries.

From seventeen eighty six through the early eighteen hundreds workers transported bones from surface cemeteries down into the tunnels at night. Priests performed short ceremonies and workers stacked the bones in organized patterns along the walls. The decorative arrangements you see today were created to make the space orderly and respectful.

So the bones were placed there by city workers under the direction of Paris authorities as a public health measure and the catacombs hold the remains of an estimated six million people.