It looks like there's a cat-and-mouse game afoot between the cops and the “cataphiles” in Paris:
[....] The cataphiles are in relentless war, “une guerre souterraine,” as Gadget calls it. Head-lamped cave-cops cruise the underground, chasing out trespassers, handing out thousand-franc fines, about $140. Manholes are soldered from above; whole teams descend hauling cement and cinder blocks — an awful sweaty job — to block off passages and seal up the rabbit holes. To no avail. Within days, the cataphiles go on the attack, using crowbars, sledgehammers, shovels, hydraulic jacks, high-powered rock drills: smashing the walls, busting the careful solders. One legendary manhole was closed and reopened and closed again 20 times in a week. The police, of course, find this infuriating; the cataphiles think it’s hilarious fun. […]
See also: Infiltration.org.