The entrance is a wet hole in the earth little bigger than an animal burrow, obscured by thorny brush in a secluded wood in northeastern France I'm following Jeff Gusky, a photographer and physician from Texas who has explored dozens of underground spaces like this one. Together we slither through the muddy hole into the darkness below. Soon the passage opens up, and we crawl forward on hands and knees. The glow from our headlamps wavers along the dusty chalk walls of the century-old tunnel, which slopes away from us down into the shadows. After a few hundred feet the tunnel ends at a little cubicle hewed out of the chalk, reminiscent of a telephone booth.