From the street level to the Catacombs, visitors descend through a series of rocky strata that date from about 45 million years ago.

During that geological era, Paris and the surrounding area were covered by a tropical sea. Several meters of sediment and mud accumulated at the bottom of this sea and became limestone over time. The level of the Catacombs corresponds to this limestone layer, which represents the period known as “Lutetian”, in reference to Lutetia, the name of Paris in Roman times. From 40 to 48 million years old on the geological scale, this bank ranges between the upper, middle and lower Lutetian. The quarries occupied the upper and middle levels, while a well nicknamed the “Quarriers’ Footbath” in the center of the circuit descends all the way to the lower level. This precise site, where the bank strata have been identified and described, has become an international reference for geological strata. 

The Lutetian layer also contains the fossilized remains of an extensive marine biodiversity. The Campanile giganteum, the largest gastropod ever found (which can measure up to 70 cm long), is one of the creatures that have been classified here. As the sea dried up, the marine fauna left behind rich fossil deposits in the limestone banks. These deposits were composed of cerithiidae and gastropods that can be seen today on the catacomb circuit. 

The Lutetian bank also gave rise to the modern paleontological classification of invertebrate animals, thanks to the work of Jean-Baptiste de Monet, Chevalier Lamark, who gathered a collection of over one thousand marine fossil species from the Parisian Basin. In 1809, the naturalist proposed a first theory to explain their evolution.

A great variety of sediments are found in Paris, and, since Antiquity, they have supplied large quantities of building materials in the form of sand, sandstone, clay and gypsum. However, Lutetian limestone, also known as “Paris stone”, supplied the major part of building stone until the early twentieth century. This high-quality stone began to be used in Antiquity for the monuments of Lutetia and was obtained from open-pit quarries dug into natural outcroppings in the Bièvre Valley. After being partially abandoned in Merovingian times, extraction started up once again in the Middle Ages, when the major Paris worksites were put in place for the building of churches, the Louvre castle and the city walls. 

As the old quarries deepened, they progressively evolved into an underground operation, with the digging of galleries to reach the banks that were furthest away in the rock. Turned pillars were put in place to support the ground, and, during the fifteenth century, a well system made it easier to bring up the stones.

The stone walling and filling method was developed in the late Middle Ages. It consists in extracting all the stone from the quarry without leaving a supporting part, then consolidating with the addition of dry stone pillars known as “piliers à bras”. Between these pillars, stone walls were erected to contain the backfill and make circulation in the quarry easier. Unfortunately, the digging of these galleries was not well-regulated by the Parisian authorities. The Paris quarries were definitively closed by the decree dated September 15, 1776, following a number of collapses in the city center.