Thanks for the link!
In the 60's the young generation started organizing crazy parties (Especially in the famous "Salle Z" or "Z Room") with hundreds or thousands of people. It has been a widely illegal and famous way to organize hazing sessions for Paris schools and universities.
Now cataphiles are all over Paris which has more than 280km of catacombs. It takes hours to go from one place to another. Some are more visited than others and easier to access. It is a common (and fun) thing to do when you are young in Paris, you can also easily find the movie theaters and get the maps from internet.
People sometimes get caught, the police is patrolling but the risk (~60€) is low.
I doubt i'd suggest it for tourists unless you have a friend who knows where to go. It takes hours to find an entrance and sometimes more than 4hours walk to find "nice" and historical areas.