Urban caver Olrik writing on concrete wall of a utility tunnel with a carbide lamp.
The flame of the acetylene gas from the lamp deposits black suit (carbon) on the wall. This is more environmentally-friendly than using spray paint to make graffiti.
Catacombs of Paris (off-limit area), about 20 meters (60 feet) underground, under Paris city center.
This is a part of the 200 Km network of ancient tunnels not opened to the public, but regularly visited by trespassing urban cavers called "cataphiles". See more information in the album description.