Near the exit of the Denfert-Rochereau metro station, there is the most ordinary-looking gray pavilion. Down from it, 133 steps of a spiral staircase descend below the surface. If you don’t have claustrophobia, heart or respiratory distress but are a lucky owner of €25 and warm clothes, then the catacombs, the most spine-chilling landmark of Paris, will reveal their secrets to you. For example, under the residential neighborhoods, there is a network of underground adits, where limestone was mined during the Roman Empire. It expands for 300 kilometers, but only two quarries are open to tourists. Limestone was first mined on the surface, but the city grew, and by the 10th century, miners had to explore the underground. Gradually, they increased the digging depth to 20 meters (approximately the height of a five-story building). Both the Louvre and the Notre Dame Cathedral were built of underground limestone. The production was stopped only at the beginning of the 18th century when groundwater flooded the quarries. The problem was solved later by concreting dangerous spaces. Twice catacombs collapsed: entire houses fell into the ground. Since then, there has been a special police unit that monitors the security of the adits, and the houses above the catacombs have to be less than six stories high.
There is a City of the Dead in the catacombs: the skulls and bones of approximately 6 million Parisians are stockpiled 2.5 meters high. At some point, the Holy Innocents’ Cemetery, where Parisians had been buried for a thousand years, failed to contain the dead: crypts and graves collapsed unattended, a layer of skulls and bones more than a meter high filled the territory of the cemetery and the basements of nearby houses. In 1786, all human remains were collected, thoroughly washed, disinfected, and sent to the catacombs, to a specially created ossuary (780 meters of the ring adit, decorated with death-focused statues and bas-reliefs).
During World War II, in Nazi-occupied Paris, there was a Nazi command post in the catacombs. As it turned out later, the French Resistance command post was located 500 meters away from it. As the sound travels badly underground (therefore, don’t leave the tourist path in the catacombs, there is no service there!), the hostile parties learned about their neighborhood only after the war.