Gusky points at some graffiti in the Cite Souterraine, or Underground City.

Jeffrey Gusky, left, escorts a visitor through the Cite Souterraine.

Gusky points out an engraving to a visitor. He began photographing the site last December.

In medieval times, villagers hid from marauders in this cave.

Gusky examines an etching left by a WWI soldier.

A sign engraved with names directed military occupants to various locations in the Cite Souterraine cave.

Nearly 2,000 century-old inscriptions have recently come to light in Naours.

A historian says the graffiti "provides insight into how (soldiers) found a sense of meaning in the conflict."

The site's proximity to the Somme battlefields, where more than a million men were killed or wounded, adds to the discovery's importance.