From "Demolition of the Paris Metro" on sleepycity.net...
The Paris Metro and the service it provides are deeply intertwined into the fabric of the city. As the 4.5 million passengers who ride it every day will probably attest it's the quickest way around whether it's for work, for play or both. The metro's distinctive art-nouveau style is unmistakable and the plant like green wrought iron entrances topped with the orange orbs and Metropolitan signage designed by Hector Guimard which sprout up all over the city lead one down to the gleaming white tiled platforms to be whisked away around Paris. On my first trip to Paris I arrived into Gare du Nord and entered the dense maze that is the metro. Despite the crowds, the noise and the distinct odour of piss, I was in love. The kind of love which inspires one to risk life, limb and deportation to get up close and personal.
Ghost stations
Before developing a deeper appreciation of the system we were drawn to the abandoned stations. Some of these seem totally abandoned and haven't been reappropriated for other uses, some have become RATP storage and others, even more rare, were never even open to the public. With time we would conquer them all.
Raccord Tunnels
In addition to the stations … the system is full of raccords, or linking tunnels, which span between lines to enable easy movement of the rolling stock.
Attendant Risks
With time the tunnels become repetitive, the junctions similar and the abandoned stations seen. Cliché as it may be, eventually the metro becomes about the experience and the adventure, more a journey than a particular destination. The journey is a conflagration of uncontrollable variables which conspire to make it unpredictable and dangerous. But that's the fun of being within a live system and as they say, there's never a dull night in the metro.
Oddities
With the risks accepted, ghost stations done, raccords run and trains encountered one begins to develop an appreciation for the oddities in the system and begins to comb it, seeking out the weird, the undocumented and unknown places. These places will always draw back those who have a stronger interest in the metro than collecting the set of abandoned stations.
Visit http://www.sleepycity.net/posts/252/Demolition_of_the_Paris_Metro for the complete fascinating account and amazing photos.