The Inner Circle is one of the best kept secrets in Paris. It sounds like an exclusive club, but it's actually an out-of-use railroad track that forms a circle around the inside of Paris. Most of the sections are closed off, out of use, and overgrown. They're inaccessible to the public, which makes them even more desirable for urban explorers with no respect for boundaries.
I discovered the Inner Circle (French translation: Petite Ceinture) on accident. There's a section open near my apartment, and while I was wandering around one day, I happened to stumble upon it. Needless to say, my curiosity was piqued, and over the next few weeks, I traveled around different neighborhoods of Paris, sneaking into various sections that were overgrown and out-of-bounds.
I had quite a few interesting encounters along the way. On one occasion, I wandered down a dark tunnel that stretched at least 100 meters, without a flashlight, clutching a sharp rock in case anyone caught me by surprise. There are stories of skinheads and homeless people emerging onto the railroad from secret entrances to the catacombs. Alas, someone did catch me by surprise, but I think I caught them by surprise, too: three young men were quite shocked to enter the tunnel and find a half-crazed American clutching a sharp rock.
We became friends and ended up exploring the railroad tracks together for the next few hours.
A few stretches of the railroad are accessible to the public, and they have been converted into public walkways. Out of respect for history, the old sections of railroad have been preserved throughout the renovations, and pedestrian paths run alongside the former railroad tracks.
All in all, the petite ceinture was one of my favorite parts of Paris. Next time I'm in the City of Lights, I'll set aside time to explore the entire thing. [Quick side note: the outfit pictures were taken a few weeks ago, when Paris was still quite warm!]