Now THERE's a hobby....
.
Who couldn't use a 'home made submarine'?
.
Hopefully he didn't build it in his basement (like all the guys years ago that would build aeroplanes in theirs, and then make the news when faced with the obvious...), the excavation of your yard to get it out would definitely raise eyebrows with the neighbours.....

I wonder if he made any torpedoes or icbm's? After all what good is a sub without the attendant weapons?