Petite Ceinture | A section in the 16th arrondissement was incorporated into the RER regional network system in 1988, and in 2008 a section between the Porte d’Auteuil and the Gare de la Muette was opened to pedestrians as a nature trail. The rest is officially off limits to the public. However, its accessibility from nine arrondissements makes it popular with urban explorers. The catacombs of Paris even have their easiest entry point from one of the tunnels on the Petite Ceinture.
