Catacombs of Paris

Centuries of quarrying limestone to build Paris itself left a honeycomb of tunnels beneath the Left Bank, largely forgotten until the late 18th century, when the city faced a very different underground crisis. Its cemeteries, above all the vast Cimetière des Innocents, were so overfilled that decomposing remains were contaminating nearby wells and even bursting into neighbouring cellars.

Starting in 1786, city officials undertook the grim task of exhuming millions of skeletons and moving them, by torchlit cart and mostly under cover of night to avoid alarming residents, into a consecrated section of the old quarry tunnels. Cemetery after cemetery across the city was cleared this way over the following decades, gradually filling the tunnels with the remains of generations of Parisians, including some who died during the Revolution.

Rather than simply stacking the bones out of sight, workers arranged femurs and skulls into deliberate walls and patterns, some of them genuinely decorative, turning what began as an emergency sanitation measure into something closer to a monument by the early 19th century.

Around six million people are estimated to rest here — far more than the roughly two million living in Paris today — yet what's open to visitors is only a sliver of the whole: about 1.5 kilometres of a network that stretches some 200 kilometres under the city. The rest remains officially off-limits, threaded by a small, secretive subculture of urban explorers known as cataphiles who slip in through unofficial entrances.

This is, quite literally, a place of the dead, and it's worth visiting in that spirit — fascinating and eerie in equal measure, but not a novelty stop. The one-way circuit is reached by a narrow spiral staircase and involves a fair number of steps down and, later, back up, so it isn't well suited to claustrophobia or limited mobility.

Tickets are timed and genuinely popular, so book ahead rather than counting on walking in. It's noticeably cool underground year-round, so bring a light layer even if it's warm on the street above.