During World War I an estimated one tonne of explosives was fired for every square metre of territory on the Western front. As many as one in every three shells fired did not detonate. In the Ypres Salient, an estimated 300 million projectiles that the British and the Germans forces fired at each other during World War I were duds, and most of them have not been recovered. In 2013, 160 tonnes of munitions, from bullets to 15 inch naval gun shells, were unearthed from the areas around Ypres.

Unexploded weapons—in the form of shells, bullets, and grenades—buried themselves on impact or were otherwise quickly swallowed in the mud. As time passes, construction work, field ploughing, and natural processes bring the rusting shells to the surface. Most of the iron harvest is found during the spring planting and autumn ploughing as the region of northern France and Flanders are rich agricultural areas. Farmers collect the munitions and place them along the boundaries of fields or other collection points for authorities

Iron harvest - Wikipedia