There is a link to a Sydney Morning Herald article about the underground city (caves) at Naours, the inscriptions left by diggers and also the inscription left by Charles Edward Fitzhenry during the war:

Search: The names of 731 Anzacs found in cave under the WW1 battlefields of France

There is some background information on Charles Edward Fitzhenry, thanks to Lucie:

Search: Charles Fitzhenry of the 9th Battalion AIF - a search for living decendants (Fitzhenry blog)

We will be visiting the caves with the archaeologist mentioned in the article, Gilles Prilaux (again, thanks to Lucie!)

Search: citesouterrainedenaours

PS: Thanks also to Jo Fitz-Henry for managing the Fitzhenry blog, and more importantly the countless hours of research on all the Fitzhenry families around the world, including my own.