It struck me as ridiculous when I heard the story, but I can somewhat see how it happens -- first, civil-war era means that it is /old/, which makes it seem les

It struck me as ridiculous when I heard the story, but I can somewhat see how it happens -- first, civil-war era means that it is /old/, which makes it seem less dangerous unless you know about the age->instability relationship with explosives; second, it looks like an inert ball of iron, so there is nothing that really screams 'hazardous' to an uneducated viewer.