There are also the mines that were placed to destroy trenches. Truly colossal quantities of explosives that were placed at the end of tunnels. Not all were exploded.
Yes, these are mines, built by miners (the people that dig tunnels to extract minerals). During the war, they dug mines under enemy lines and filled them with tons of explosives. Not all were used and the location of some unused ones has been lost [1]. One of them blew up from a lightning strike in 1955[2].
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.