The Worst World War wins the barrage game because the lines were often static. I spent a week with a French EOD team once back in the day. They were collecting unexploded ordnance from both war. The shells would go deep, then the annual freeze would push them toward the surface. Mostly into farm land. Farmers would collect the shells and leave them at "the usual place" wherever that was. Le EOD would safe them and move or explode them in situ. There may still be a lot of those still undiscovered. Bad when a tractor plows into one. Had to use my medic training once. Wasn't sufficient.