La Petite Ceinture
A little secret of Paris is La Petite Ceinture, an abandoned railway surrounding Paris, circling the Périphérique. The reason for it's abandonment is that it became obsolete after the introduction of the métro and has been disused since 1934. As you can imagine it is now overgrown and wild, home to "more than 200 species of plants and more than 70 animal species" according to ParisInfo.com
Parts of La Petite Ceinture have been opened to the public in the 12th, 13th and 15th arrondissements, with the section in the 12th being wheelchair and pram friendly. However, the nicest and most interesting sections to explore are the parts still closed off. It is technically forbidden to enter La Petite Ceinture, that being said I have never seen it being policed or people being stopped from entering.
The easiest place to access La Petite Ceinture is from within Parc de Buttes Chaumont, in the 19ème arrondissement. Take métro ligne 7bis to Buttes Chaumont and use Google maps to find Rue de Crimée, on the east side of the park. Enter the park from this road, and you will be walking along Avenue Crimée within the park. If you continue along Avenue Crimée you will reach a bridge, and if you look down here you will be able to see La Petite Ceinture (and probably a few people!) In order to enter it yourself you have to walk down the banks either side of the bridge (there are gaps in the fence on both sides) and walk along until you find a gap or break, of which there are many! There are of course many other places from which to enter, but for your first time this is the safest and easiest entrance to find. (You can find the exact location of the entrance here using my interactive map.)
Once "inside" La Petite Ceinture it is all yours to explore! I would recommend taking torches to walk through the tunnels, and even in Summer dress warm as the tunnels are cold and longer than they look! La Petite Ceinture is used for all kinds of things, by all kinds of people. There you can find street artists, rebellious teenagers, squatters, people having bbq's, homeless people, school children and cataphiles (we'll get onto that later)!
Obviously, being so secluded you do need to be careful here when you go exploring, there are dangerous things that go on here, and there are dangerous people. I would never recommend going alone, that being said I have never encountered a problem, during my explorations and it really is a great place to experience.
For example, La Petite Ceinture is home to many many entrances to the Paris Catacombs, which are underground ossuaries, and also connect to the carrières de Paris, the mines, so essentially from La Petite Ceinture you have access to all of Paris' underground tunnels. Again, these are forbidden to enter, and if you do you should proceed with due caution as they are not maintained and cave ins do happen. If you want to explore I recommend seeking out a "Centiphile" which is someone who has great experience of the Catacombs and can help you navigate your way through. Or, alternatively, there is an area of the catacombs open to the Public, accessible via Denfert-Rochereau, and costs around €13.
The sorts of things that go on within La Petite Ceinture and in the forbidden tunnels of Les Catacombs has become the stuff of Urban Legends. There are rumours of serial killers hiding out here, Nazi gatherings and masonic meetings. This may be true or not, however, somethings that do happen and people have written about their experiences of are illegal Raves, underground dinner parties, clandestine movie screenings... there are definitely fun and exciting things going on here but I'd go and find out for yourself!