When I was studying in London in the mid-1960s, it was almost a daily occurrence for some construction crew to turn up a buried German bomb. The Metropolitan Police had a special unit for it, as I recall. I guess we are out at the tail end of the distribution now.
Anyone know whether farmers in Picardy are still finding unexploded munitions from WW1? They were in the 1970s, according to Paul Fussell’s book.