At the end of the 18th century, rampant disease in the Les Halles neighborhood caused by the adjacent Cemetery of the Innocents led to the graveyard being entirely exhumed. Fearing that wells might become contaminated by seepage from the cemeteries, the decision was taken to no longer permit burials within the city itself. The cemeteries of Père Lachaise and Montmartre date from this period; both were, at that point, 'outside the city' if not outside the walls.
In 1785 it was decided that the bones were to be moved to the building stone quarry under the Montsouris plain in the south of Paris. On April 7th, 1786, after being properly converted and readied, the quarries were consecrated and became the principal ossuary of Paris.
Until 1788, cartloads covered in black drapes, escorted by priests chanting the Office For The Dead, crossed Paris by night to deposit their remains.
Originally, the bones were simply dumped. Louis-Etienne François Héricart de Thury, Engineer-in-Chief of Mines, had the bones rearranged artistically and made the galleries accessible to the public in 1810. The result is what you see here: thousands of skeletons, their constituent parts sorted and neatly stacked, the various collections marked to indicate which cemetery originally held the remains.