Fishermen coming in with old naval mines in their nets are a pretty common occurrence in Denmark. Those things are just crazy and it seems like dumb luck that almost no people aren't killed by them.
Well they didnt go off when they were supposed to and haven't gone off in 80 years -> there's a good chance there's something about them preventing them from going off.
The main danger is that disturbing them could remove the inhibitor. Also, age can make them unstable.
This is why we blissfully ignore the known unknown bombs that are everywhere in Europe but are extremely careful when we discover and try to remove one.