What Lies Beneath

A common expression in France is that "Paris est un Gruyère", meaning that the city has been built over a terrain riddled with holes. This phrase is incorrect dairy-wise as Gruyère does not have holes, but geologically very accurate. Over the centuries parts of the city have regularly dropped down into the depths below, but is it still something the city’s inhabitants need to be concerned about today, and who needs to worry about this?

On the 17th December 1774, a 300 metre length of the Route d’Orléans near the Enfer (Hell) entrance into the city (today the place Denfert-Rochereau) fell into a 25 metre deep hole. Three years later, a house nearby fell 28 metres into the ground. Following these incidents, an engineering department was set up with the goal of investigating and mapping this subterranean landscape. This entity still exists today and is based in one of the original Enfer gateways, sitting directly above the entrance to the city catacombs.

What they found was that the city of Paris is built above a series of ancient mines, quarries and passageways that stretch across a total of 300km. Some of these are still visible, notably at the catacombs, but also in open air sites at the Parc de Buttes Chaumont and at the old brick making works at Vaugirard. The modern city though has added additional tunnels, with the Metro and RER systems cutting through these old passageways, alongside sewerage and gas pipelines.

Such ‘secret’ passageways have naturally attracted active groups of city explorers who ignore official rulings that ban them from entering these tunnels, and special teams in the city Police force whose role is to patrol the tunnels. Both groups sometimes make some very surprising finds, with the Police recently discovering an underground cinema and restaurant under the 16th Arrondissement.

The amateur groups, known as cataphiles, are secretive about their membership, places of exploration and most importantly, which entrances they use to access this underground world. Coming from a very wide range of backgrounds, from artists to professors of history, the motivations of each is also very disparate. Some descend below the city surfaces to explore, some to create, some simply to find silence. Many do so regularly, with first time visits very difficult to organise. Indeed, these people are well aware of the dangers, the greatest being simply getting lost in the endless networks of tunnels.

The official explorers of these tunnels are the employees of the Inspection générale des carrières. Somewhat exasperated with the amateur explorers who glamourise this universe, the inspectors are more concerned with mapping and predicting the dangers. The importance of their role is only noticeable when problems occur that they have not seen coming. In 1995 an entire street (Rue Papillon) was evacuated when a tunnel collapsed during construction work for a new RER train line, and more tragically, 21 people were killed in 1961 when six streets collapsed in the suburb of Clamart.

A scene underground in Paris.

An official service exists for inhabitants of Paris to see if their building is considered to be at risk or not

. If you are a property owner in the city, this information could be very important to you, as it has been decreed that individuals do not only own a share of the building in which they live but also a certain distance beneath the ground of the building. Landslides are still common beneath the surface of the city, and unless this problem can be attributed to public works, the building management group will probably have to pay for any damage caused to the structure.

This is an interesting issue in cities. In London where space is at a minimum, property owners have begun digging down into the land beneath their properties. When planning laws make extensions almost impossible to contemplate, the only way to build is down, and as The Times newspaper reports, people have been adding swimming pools and tennis courts beneath their houses.

In Paris, where the Metro runs only a few metres beneath the surface of the city, this would be impossible, but home owners should still check how risky the land beneath their property is. In any case, for somebody who is interested in the invisible side of cities, I find it fascinating to contemplate a world that is truly invisible and yet so important. Fortunately, my building is seemingly not at risk from a catastrophic dive down into an ancient underworld.