My new favorite thing about Paris: an underground group of cultural guerillas called the Untergunther (!!) who sneak around and restore monuments.

Klausmann and his crew are connaisseurs of the Parisian underworld. Since the 1990s they have restored crypts, staged readings and plays in monuments at night, and organised rock concerts in quarries. The network was unknown to the authorities until 2004, when the police discovered an underground cinema, complete with bar and restaurant, under the Seine. They have tried to track them down ever since.

From this article.