Paris' Petite Ceinture is an abandonned railway inside Paris that goes all aorund the city.
It is closed to public and hardly accessible so you normally cannot go there... But you may find entries here or there and climb up to this forgotten paradise!! Silence, nature taking it's rights back on the city, no or very few people, freedom!!).
Then you can walk there during hours, following the old railways and enjoying being in another world than the usual crowdy, stressy, crazy Paris...
Will definitely go there again to discover some new magic places,abandonned train stations, tunnels, etc... However if you go there try to be at least 2 persons... You never know who's hanging around there... but that's what makes it even more adventurous right?? :)