Underground Paris

It looks like underground Paris is even more interesting than underground London:

The Paris underground, often referred to as the catacombs, has been luring curious visitors for centuries. The City of Lights is built atop a vast realm of darkness: enormous gypsum and limestone quarries that were mined beginning in the 12th century for the construction of Notre Dame, the Louvre, and other edifices. Burrowed haphazardly beneath the surface city, these quarries became increasingly unstable over time. When a street collapsed in 1774, Parisian authorities investigated the galleries and reinforced weak areas. As they did, the investigators marked the tunnel walls with the names of the corresponding ground-level streets. These two-century-old signs are still used for navigation.

The freshly mapped underworld would soon have many uses. From 1785 until the 1880s, the quarries received bones from Paris's overflowing cemeteries—the public Les Catacombes museum, housed deep inside a blocked-off section of the quarries, alone contains the bones of some six million citizens. During World War II, the passages were occupied not only by the French resistance but also by Germans, who left their traces in a military installation called the Bunker Allemand. Since then, artists, performers, graffitists, and others have added to the catacombs' multilayered history. "Regardless of where your research takes you, there are always new things to discover about subterranean Paris," says Ingmar Arnold, a Berlin-based underground historian. "Wherever you walk, you can never be sure you're not passing across something mysterious—behind every corner there could be a great secret."

One of these secrets was unexpectedly revealed in September 2004 when the Paris police discovered an illegal cinema beneath the Palais de Chaillot. Patrolling officers had stumbled across the hidden amphitheater, fully equipped for movie screenings. For an illegal setup, it was remarkably sophisticated: Next to a screen and projector sat a bar and restaurant outfitted with several telephones and Internet access. The complex was protected by a closed-circuit-TV security system that set off a recording of barking dogs whenever an intruder passed by.
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We crawled through a narrow corridor that dripped with mold and entered the heart of this ossuary. There, a central room, filled with stacks of bones, opened onto small tunnels on every side. The entire area was saturated by an ocean of broken skeletons. It was impossible to move without crawling over piles of bones several feet deep—they crackled beneath our weight. In one eerie chamber, a sculpture made of femurs rose from the scattered heaps. Only John and I climbed into that central room; turning off our lights, we crouched on bones in utter darkness, each paying respect in our own way.

The article refers to Carolyn Archer's book Paris Underground, which contains photographs of some of the underground locations. The underground movie theater is described in a 2004 article in The Guardian.