Cité station, Métro de Paris, France. Public Domain.
View to Montmartre, Paris, France. Public Domain.
View to tour Eiffel, Paris, France. Public Domain.
Louvre, Paris, France. Public Domain.
Map of underground quarries, Paris, France. Public Domain.

The Île-de-France (Isle of France) is the region of the capital Paris and also called the Région parisienne (Paris Region). It is located in a basin filled with Mesozoic sedimentary rocks. While there are thick layers of limestone underground, the relief does not allow underground drainage, and so there is no karst and no caves. But the limestone was quarried underground for centuries to erect the buildings on the surface. The underground sites in this region are all artificial, and most of them actually use the abandoned limestone quarries.

Like most cities, at least in Europe, Paris has huge subterranean cavities, built during 2000 years of history for many purposes: quarries, religious hideout, wine cellars, subways and bunkers. Most of the excavations are located at the base of the three "mountains": Montparnasse, Montrouge and Montsouris. There are approximately 300 km of galleries all together, not all of them are connected. The most extended connected system of tunnels is located under the 5th, 6th and 14th arrondissement, and is about 100 km long.

Limestone quarries exploited between the 13th and 18th century the calcaire grossier (coarse limestone) which was used as building stone. They also mined pierre à plâtre (plaster stone), a lower quality limestone which was purned in kilns to produce plâtre (lime) of excellent quality. The limestone layer was named Lutécien, after Lutetia, the Roman name of Paris, which we all know from Asterix. Actually, Lutetian is the international geologic term for this age. It was deposited between 48 and 41 Ma during the Eocene, in the Tertiary, in a warm and oxygen rich sea. The limestone contains many fossils, mostly bivalve molluscs or molluscs with elongated shells. Here in Paris, this limestone layer is found between 37 m and 80 m asl.

Limestone quarrying was started by the Romans in the 1st century, in the Bièvre Valley. Until the Middle Ages, the amount of quarried limestone was rather low and the quarrying was in open casts. When the high quality limestone became scarce on the surface, the quarrymen dug deeper and deeper until they finally went underground. The quarries followed the rock qualities which were needed for buildings, when there was no more stone to extract, quarries were closed and abandoned. The rocks were mined in a simple but rather safe technique called room and pillar, where only a part of the rock is removed and the rest remains to form huge pillars supporting the ceiling. This works well if the pillars are big enough, but the rock is precious and especially illegal quarries tended to leave too small pillars which collapse decades later.

There were other rocks below the city, which were mined, but to a much smaller extent. Green marl and clay were used to make tiles, bricks and pottery. Fine gravel and sand, known as Fontainebleau sand, was used for glassmaking and foundries. And underneath present-day rue Saint-Gothard two centuries ago, a large deposit of coal was mined.

Today there are hundreds of kilometers of abandoned quarries which are not a tourist site, often dubbed Catacombes de Paris, although the term is completely wrong. And they are not hard to find, there are virtually hundreds of entrances. And if you have found the entrance to a quarry, it's easy to explore all parts of the quarry, ther is not a single passage which could be walled or closed by a door. The room and pillar technique left a huge chamber with hundreds of pillars creating a chess board of passages in two main directions. Even the ossuary is not a separate structure, it's a section of a huge quarry. Urban explorers, in France they are often dubbed troglodytes (cave dwellers), explore those spaces, take pictures, and have parties in the underground. There are raves, restaurants, a cinema, and all kinds of subculture you may imagine. For example, La Mexicaine de Perforation, an artistic movement with the intention to improve Paris by illegal occupation and restoration. Regular patrol of the police is futile, as there are virtually hundreds of kilometers of underground tunnels, nevertheless there is a special Catacombs police, colloquially called cataflics.

The government has restricted the visit of the catacombs because of safety reasons. The safety of the visitors, the safety of the catacombs, and the safety of the city above. And actually, there are downsides to those illegal visits. In 2009 the bones piled orderly in stacks were vandalized, bones and skulls were scattered along the walking paths. As a result, the catacombs were closed for three months to the public. And in 2017, thieves broke into a cellar from the catacombs and stole wine worth over 250,000 euros. Only the spectacular cases are published, there are obviously a huge number of incidents.