OT-Messines Ridge

[Sanjuro]

Anyone know how long Ammonal remains potent? The reason I ask is, I have just been reading about the use of mines at Messines Ridge in 1917- of the 21 mines dug, only 19 blew as planned (the largest man made explosions in history, until the development of atomic weapons). The 20th blew up when struck by lightning in 1955- fortunately only killing a cow.
Depending on which source you believe, the 21st mine may have been
(a) dismantled before the battle because German troops were close to finding it
(b) found in the years since
or (c) still unaccounted for. That would make a nasty surprise for a group...