Just a point:
How much of the old ordinance discovered IN UK by detectors, construction work, magnets etc has exploded spontaneously in say the last twenty years.
I know that a bomb blew up in an “unanticipated explosion” in 2023 but it was already being emptied by the experts. (No one hurt)
I’m not suggesting that I wouldn’t be very cautious not to say scared if I found any but I am not aware of any injuries caused by WW2 and older explosives in the last few decades.
I imagine that magnet fishing is a very interesting pastime. My one attempt was practical rather than interest but the anticipation was there. I bet most bridges over canals etc are well fished by now.