Six million Parisians lie in the ossuary, and it occupies barely one percent of the tunnels beneath the city. The rest is quarry. Since Roman times, masons cut Lutetian limestone from the bedrock south of the Seine, and that stone became Notre-Dame, the Louvre colonnade and the hôtels particuliers of the Marais. Paris Catacombs Scheduled Access with Seine River Cruise tours begin above one such quarry, at Denfert-Rochereau, twenty metres over a void the city dug for itself. The crisis came in 1780. The wall of the Saints-Innocents cemetery gave way, spilling remains into a neighbouring cellar in Les Halles. Six years later, the crown ordered the graveyards emptied. Bones travelled south by night in black-draped carts, priests chanting behind them, and were tipped into the galleries through a well on the rue de la Tombe-Issoire. Charles-Axel Guillaumot, first Inspector General of Quarries, had already spent a decade propping the same tunnels with masonry piers to stop streets from swallowing houses. The dead were stored in a structure built to save the living. Louis-Étienne Héricart de Thury turned storage into architecture after 1810. He stacked tibiae and skulls into retaining walls called hagues, set them in crosses and hearts and barrels, carved doorways, and hung Latin verse along the route. The threshold inscription still reads Arrête, c'est ici l'empire de la Mort. Nearby stands the Port-Mahon relief, a Minorcan fortress cut into the rock by a quarryman named Décure, beside the well the workmen used to mix their mortar. The galleries opened to the curious in 1809 and now sit within Paris Musées, alongside the Musée Carnavalet. Temperature holds near 14°C year-round, and humidity, footfall and vibration are monitored constantly, which is why a catacombes de Paris visit is capped and timed rather than open-door. Above ground, the same geology shapes the city: hollow bedrock under the 5th, 13th and 14th arrondissements, surveyed still by the Inspection Générale des Carrières across some 300 kilometres of galleries closed to the public. Only about 1.5 kilometres are legally walkable. That single lit corridor is the reason a skip-the-line Catacombs visit pairs so neatly with the river: one route reads the city from below, in stone and bone, while the other reads it from the water, past the Île de la Cité and the UNESCO-listed quais. Paris Catacombs Scheduled Access with Seine River Cruise tickets sit at that hinge between the two Parises.