>>49621
I bet it was used a lot. If there is a naturally or artificially occurring hole, a soldier will use it... :^)
Tunnels are used since forever for military purposes. Soviets also loved them. Those and railroad. Sometimes combining them.
We have an old school fortification, it had tunnels all over, Russkies used it during the years of occupation and they widened the tunnels at places so tanks could roll through.