more than 2,000 perfectly preserved inscriptions were found on the walls of an underground cave in naours , france .
they were left for posterity by young men facing the horror of trench warfare a few dozen miles away at the somme .
experts believe the bored young soldiers used their spare time to visit the caves , which are well known locally .
only weeks later they were sent to the somme battlefields , where more than a million men were killed or injured .