The caves are only accessible by small spiral stairs in the Bouzincourt church seen below.
Some 12 yards under the surface, the caves were used by locals as early as the 17th century to store food and shelter their families and livestock.
"They knew that they may be about to die," Gusky told Reuters in 2015.
"We all want to be known, we all want to feel like our lives matter," Gusky said. "And so here on these walls we see them writing their last message to all of us, not knowing if anyone would ever see it."
Many of the shapes carved out of the cave walls are designed specifically to allow a postcard to be inserted there, Gusky said.
next slide will load in 15 secondsSkip AdSkip AdOthers are inscriptions of the name of a soldier, often followed by details of his injuries. Of the 829 names recorded in the caves, around 500 are of Canadian soldiers.
While others are crudely carved flags or hearts.
There are even relics not carved in the stone.