[hanamizu]

I read that there are at least a couple of unexploded ‘mines’ containing many tons of explosives that have not yet been found. Their locations were lost decades ago.

[robowombat]

There is one of the huge mines Gen Plumer had put in place for the Arras operation in March 1917 still unexploded. These were really big mines. When detonated the explosion was clearly heard in the SE UK. There is a book,’Ear Underground’ which covers this episode in fascinating detail.