The Secret Catacombs of Paris -

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. The Guardian's architecture and design correspondent Jonathan Glancey investigates the underground maze of Paris, revealing a mysterious and intriguing history.