[Someone]

AFAIK, no mines were used at Verdun. Also, the ones used at the Somme all exploded (https://en.wikipedia.org/wiki/Mines_on_the_first_day_of_the_...)

I guess you are thinking of https://en.wikipedia.org/wiki/Mines_in_the_Battle_of_Messine.... One of them exploded in 1955, a few others may yet explode or may be fairly harmless by now.