The "Z" Room, below l'Ecole Normal Superieure on the rue d'Ulm, in Paris' 5th district. (Photo by Olivier Faÿ/Sygma/Sygma via Getty Images)
Ossuary, in honor of Halloween night. (Photo by Olivier Faÿ/Sygma/Sygma via Getty Images)
The Cellier, below the rue Sarrette, in the 14th district of Paris. This room was once a brewery. (Photo by Olivier Faÿ/Sygma/Sygma via Getty Images)
The shadow of a man exploring the watery catacombs of Paris below Rue de l'Aude in the 14th district of Paris, flooded gallery and pillar. (Photo by Olivier Faÿ/Sygma/Sygma via Getty Images)
Inspection gallery below the street of Assas, in the 6th district of Paris, formerly called the rue de l'Ouest. (Photo by Olivier Faÿ/Sygma/Sygma via Getty Images)
