Filling is mostly to reinforce zones threatened by collapse. Entrances are secured mostly by sawing off the ladder rungs and welding the manhole covers.

> ambiance" was not as romantic as you might think from reading the article

Romantic to people who enjoy mud !

> cataphiles were very hostile to other visitors

The small groups who go down to party in what they consider their own private space can indeed be a bit noxious, with reactions such as lighting up smoke generating fireworks to drown the a bit of the network... I won't forget the adventure of finding our ways with one foot visibility.