aerial photographs of World War 1 practice trenches at Bodelwyddan North Wales 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 did his training in Anglesey. They built training trenches in North Wales followed by real ones on the Somme. Its even possible he had a hand ( or spade ! ) in these as they were certainly not confined to Anglsesy. one exercise was building a Pontoon bridge at Conwy. He explained how the trenches 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.