Why the Western Front's iron harvest continues

Why is unexploded ammunition still being recovered from the Western Front?

Because armies fired shells in such vast numbers that even a very low defect or failure rate left tens of thousands of dangerous rounds behind.

A hypothetical defect rate of 0.5 percent means one bad shell in every two hundred. Applied to twenty million rounds, that still leaves one hundred thousand defective shells. At the enormous volumes of First World War ammunition use, even a small failure rate therefore produces a large remaining hazard. The same one-in-two-hundred figure is later offered tentatively as a wartime average, with an explicit qualification that early-war problems and particular guns could give worse results. It should not be read as a measured rate for every army or weapon. Continuing recovery of unexploded ammunition reflects scale as well as variation in manufacturing and reliability.