Beneath the streets of the 14th arrondissement lie the ossuary galleries of the Paris Catacombs: 2 kilometres of former limestone quarry tunnels lined with the bones of approximately 6 million Parisians, transferred here in the late 18th century when the city's overflowing cemeteries became a public health crisis.
The descent involves 130 narrow spiral steps; the temperature underground is a constant 14°C year-round. The bones are arranged with surprising care — skulls and femurs stacked in geometric patterns, occasionally decorated with crosses and hearts made from bones. The effect is genuinely affecting rather than merely macabre, and the silence is profound.
Book weeks ahead — this is one of Paris's most popular attractions and the daily ticket allocation sells out quickly. The official tour covers only a fraction of the full tunnel network; urban explorers (cataphiles) still navigate the illegal extended system, but that is a different and considerably more dangerous proposition. Above ground, the small museum at street level is worth ten minutes before you descend.