The 100 year-old German grenade

Sometimes truth really is stranger than fiction.

Imagine the following scenario: During WWI, factory workers in Germany assemble a Granatenwerfer grenade for use against troops of the Allied Powers. 

A century later, that same grenade, still unused and undetonated, turns up in a river in Grand Rapids, Michigan–one century, two continents, and an ocean away.

Oh, and a man pulls it from the river with a magnet!

If a novelist included such a plot device in a story, readers would call it contrived. But this is exactly what happened in real life, as the above story details.  

Exactly how did a 100 year-old German grenade from WWI end up in a river in Michigan?

I don’t think that anyone will ever know the full answer to that. But it must be a good story.