Stone plate indicating underground depth... very accurately!
This plate was probably carved at the early times of the metric system, where old "feet" and "inches" measures were still used (they were a bit longer than the one used in the USA, by the way). The engineer was a bit too zealous when converting 58' 6" into metric. A bit like this other sign...
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.