The stench is the first thing that I notice as I enter the dark underworld of the Petite Ceinture, the long-abandoned more than 150-year-old railway line which traces the 32-kilometer perimeter of Paris. Some parts of the Petite Ceinture are now open to the public, but the kilometer-long tunnels are barred and you have to be sneaky if you want to explore them. I found a way in (see below), where the dank air smelled of urine, rot, and abandonment. It was the smell of a place forgotten. Moving further on, the beam of the torch on my mobile creates a bubble of light in the almost impenetrable darkness. It flashes at the stone walls on either side, and the circle of light picks out tags, scribblings, and crude drawings, as this abandoned place has probably for almost a century been witness to furtive meetings, drug use, rough sleeping, and teenage gatherings. I walk slowly, wary of deep holes in the floor and obstructions on the tracks. Everything...