Unfortunately, a huge percentage of those shells just ... didn't go off. As many as 1/3rd of them failed to detonate and embedded themselves deep in the mud. To this day there are areas in France that are completely off limits to everyone, both due to the unexploded ordinance and the amount of toxins those areas. Even outside of those areas there is a a process known as the "Iron Harvest", where local farmers will dig up artillery shells regularly; there is usually an official process for where farmers put these shells for regular disposal.