Cities of the Underworld is a documentary hosted by Don Wildman following history of a city and the underground. Literally. The documentary explores tunnels and ditches of dark places.
Sleepcity has posted numerous photographs with beautiful descriptions of the Paris underground tunnel system involving the metro.
The subway construction of the tunnels began in 1896 to provide transportation for the Parisians. "[R]esidents even went to far as to ensure the metro ran right hand side, to guarantee non-interoperability with the left hand side system in the suburbs."
via. BLDG Blog