The swath of modern-day France where the WWI Western Front was fought is still riddled with unexploded munitions. They're buried underground, but get slowly brought to the surface by freezing/thawing, just like rocks. It's so bad in places that there are national preserves that the public is not allowed to enter, and a permanent, professional group of demineurs who go around collecting the various shells, grenades, and bombs that pop up (both inside and outside of the preserves).