Shell hole

A shell hole was the cavity made by the explosion of an artillery shell.

On Christmas Day 1914, Archibald Hamish Lethbridge-Stewart and a German Soldier lay in a shell hole on the battlefield in Ypres. They were originally meant to kill each other there, but after the Twelfth Doctor made some changes to the timeline, they were both saved as the Christmas truce began. (TV: Twice Upon a Time)