Trench Warfare
Trenches were long pathways dug out in the ground, several feet deep. They were used to avoid fire while shooting by being under the ground instead of standing in an open field where you are an easy target.
The trenches weren't all good though. The trenches were festering with disease. If the gunfire and gas didn't kill you, the diseases surely would. Trench foot was very common, it would cause blisters on the feet that would tear off the skin when they popped. Many other illnesses could also be found in the unsanitary conditions of the trenches.