BBC Radio Four ran a fascinating programme on this “unknown” part of Paris. I understand it’s “Illegal” to go down to the “secret” tunnels, but you can visit the ossuary.
Listen to the programme on the iplayer if you can.
I love Radio four, so many super little programmes, I would listen to it all day if I could. Sometimes catching up isn’t quite the same, and often if you listen live then you may listen to a programme that you wouldn’t have thought you would be interested in.

Famously known as the City of Light, Paris is a diverse metropolis rich in architecture and steeped in history. But it has a dark alter ego that lies 30 metres under the ground, mirroring centuries of bloody wars, revolutions and riots on the surface. For Paris is porous – built on 177 miles of tunnels that were formed when limestone and gypsum were quarried to build the capital. Most people are only aware of just a tiny fraction of these tunnels – the world famous ossuary known as The Catacombs. The authorities have tried to keep a lid on the full extent of the labyrinthine remainder for hundreds of years. But there are little known entry points everywhere – in basements, in train stations, cellars and sewers. Throughout history, invaders have always found a way in, whether they were fighting Prussian soldiers, fleeing royalty of the French Revolution, the Nazis or The Resistance. Today they’re home to the cataphiles – urban explorers who use the tunnels as an art space, a music venue or even a clandestine meeting point for secret societies.