Secrecy is, of course, essential for Kunstmann and other members of the UX.

CNN: Bone people: The explorers of the Parisian catacombs

We first met Lazar Kunstmann - not his real name but the one he insists on using -- in an anonymous Parisian bar.

CNN: Bone people: The explorers of the Parisian catacombs

Over a beer Kunstmann explains what motivates the UX.

CNN: Bone people: The explorers of the Parisian catacombs

Before we leave him Kunstmann tells us that the UX have several more projects in mind but, unsurprisingly perhaps, he declines to elaborate saying future plans have to be kept under wraps.

CNN: Bone people: The explorers of the Parisian catacombs