Six million Parisians rest beneath the city's fourteenth arrondissement, and almost all of them arrived after 1786, carried by night in black-draped carts. The Catacombs of Paris were not designed as an ossuary. They began as Roman-era limestone quarries, the same pale stone that faces Notre-Dame and the Louvre, hollowed out over centuries until the ground above began to collapse into the voids beneath it. When the Cimetière des Innocents finally failed in the 1780s — its mass graves overflowing into neighbouring cellars — the monarchy ordered the dead removed. The transfer took twelve years. The bones were stacked into the abandoned galleries by quarry inspector Louis-Étienne Héricart de Thury, who arranged femurs and skulls into walls of deliberate geometry, added Latin inscriptions above the doorways, and opened the site to the curious in 1809. What began as sanitation became architecture. The modern route runs roughly twenty metres below street level, reached by 243 steps, and covers about 1.5 kilometres of galleries at a constant 14°C. Above it, the city keeps its lighter attractions: a seine river cruise near eiffel tower departs from the Port de la Bourdonnais, threading past the Pont Alexandre III, the Île de la Cité and the flying buttresses of Notre-Dame. Visitors increasingly pair the two, and combined seine river cruise near eiffel tower tickets exist precisely because the contrast is the point — the same limestone appears in both directions, quarried below and dressed into quays above. The geology explains the coincidence. Lutetian limestone, laid down when the Paris Basin was a shallow sea, built the city and undermined it in equal measure. The Inspection Générale des Carrières, founded in 1777 and still operating, maintains more than 300 kilometres of tunnels beneath the capital, of which the public ossuary is a fragment. Paris landmarks visible from a boat deck — Haussmann's façades, the Palais de Chaillot, the Trocadéro terraces — sit atop that hollow record. The Catacombs matter today less as a curiosity than as a document. The bone walls preserve burial practices erased elsewhere; the graffiti includes signatures from 1781 and from Wehrmacht soldiers and Resistance fighters who used separate sections of the network in 1944. Conservation work continues on the Port-Mahon sculptures, carved by a quarryman named Décure from memory of a fortress he had seen as a soldier. Entry is 31 EUR at the full rate, audio guide included, at 1, Avenue du Colonel Henri Rol-Tanguy, 75014 Paris — an address named for the Resistance commander whose bunker sat metres away. A Paris Seine boat tour lasts an hour; the descent below lasts about the same. Together they trace one city read twice, from the water line and from the quarry floor, which is why a seine river cruise near eiffel tower tour so often ends underground.