The Catacombs of Paris

The remains of more than six million people, moved underground because the cemeteries above had run out of room.

The Catacombs of Paris are underground ossuaries built into the city's ancient stone quarries, extending south from the former city gate at the Barrière d'Enfer, the Gate of Hell. They were created to deal with Paris's overflowing cemeteries, and the work acquired real urgency after a series of basement wall collapses around the Holy Innocents' Cemetery in 1774. From 1788, nightly processions of covered wagons carried remains out of most of the city's cemeteries to a mine shaft opened near the Rue de la Tombe-Issoire.

Then the place was largely forgotten, until it became a novelty venue for concerts and private events in the early 19th century. After further renovation and the construction of accesses around Place Denfert-Rochereau it opened to public visitation in 1874. Since 2013 the Catacombs have been one of the fourteen City of Paris Museums managed by Paris Musées.

One clarification that matters underground: the ossuary is only a small section of the mines beneath Paris, though Parisians commonly call the entire tunnel network the catacombs. What you can legally visit is the ossuary.

The question of what is owed to the dead here is live. In October 2015 Airbnb ran a Halloween competition offering the winners a night in what it called the world's largest grave, and opposition politicians on the Paris city council urged the mayor, Anne Hidalgo, not to forget the French law requiring respect for the dead. The BBC covered the row.

Six million people. Go quietly.