This story originally appeared in the August 2014 issue of National Geographic magazine as "The Hidden World of the Great War ... of the trenches above. A journalist visiting one of the caverns ...

I quite like playing in the mud. But I’d probably feel different if I was a soldier in the trenches during World War One. WW1 SOLDIER JACK: Mud? Oh, we know all about mud here on the Western Front.

An antique that wouldn't look out of place in a trench during World War One: an old wind-up phone ... He says it's the safest means of communication, and that "it's impossible to listen in".