aerial photograph of World War 1 practice trenches at Iron Mills Shropshire England UK. Still clearly visible a hundred years after they were dug these WW I military practice trenches were dug by new recruits as they trained for combat in the trenches of France. My Grandfather was a Royal Engineer and built similar training trenches in North Wales followed by real ones on the Somme. He explained how they worked including building them as a zig zag so that if part of it was taken it was not possible to shoot far along the lines. You can see the layout here of zig zags and the way a forward trench is built linked to a reserve trench in case the forward one was taken.