As the war developed into a relative stalemate, each side dug extensive networks of trenches. Initially the trenches were just crude ditches, but as time went on they became permanent living quarters as well as defensive positions. Deep underground shelters were dug to provide protection to troops from enemy artillery. Sometimes the trenches ran through areas of naturally occurring cave systems and soldiers took advantage of the existing formations and became cave dwellers.
Here in the damp darkness the men could rest, write letters and enjoy a little respite from the horrors on the surface. This photo shows French soldiers resting an underground cave. Most of them are sleeping or lying down on cots lining the walls of the cave; one of the men is writing a letter home by the dim light of this cave. As bleak as this home was at least it afforded relative safety.