THE Paris Catacombs, which hold the remains of more than six million people, will close for six months from November for restoration work.

Humidity is causing some of the bones to decay, and, in some areas, stalactites are beginning to form on the tunnel ceilings owing to water seeping through.

A spokeswoman for Les Catacombes, Hélène Furminieux, was quoted in The Times as saying: “Cracks are appearing, and there is a risk of localised collapse in some places because rainwater is often acidic and can dissolve the limestone in which the tunnels were cut.”

She said that they were not at risk of a general collapse yet, but that structural reinforcements and damp-proofing were needed. “Water can take tortuous routes that are hard to trace,” she said.

The lighting in the catacombs will also be upgraded to enhance the visitor experience and draw attention to specific features, such as the inscriptions. The most recognisable of these is the words on the entrance, “Arrête! C’est ici l’empire de la Mort” (“Stop! Here is the Empire of the Dead”).

“Today, the idea of mass graves evokes massacres and a lack of respect for the dead, but this was not the case in the Middle Ages. It was only in 1805, under Napoleon, that commoners started being buried in individual coffins,” Ms Furminieux said.

The underground networks and ossuaries, which are 65 feet beneath the French capital, were first opened to the public in 1809. Today, visitors can see just a fraction of the tunnel network: only a 1.5 km circuit is open to the public. Some people reportedly know about secret entrances through basements and sewers.

Approximately 550,000 people visit the Paris Catacombs each year. It is thought that the public body that manages the site, the Paris Musées, will lose as much as €1 million per month during its closure. The city council will provide a €4.4 million subsidy, The Times reports.