U-Boat "Cruiser" Type XIB
 

Apparently this sub never existed or was cancelled?

But then again, some claim to have found one? Anyone know anything about this? Is it a hoax?

http://www.combatsim.com/memb123/htm/feb99/type-xib.htm

No Type XI boat was ever completed AFAIK, all canned with the outbreak of war. If one had put to sea, I'm sure someone would have heard of it.

Though, given more evidence, it could theoretically be true.

Well, this is why I included that link.
According to it, there is some speculation (I hate to use the term 'evidence') that a recovery op took place off the American east coast where such a titan was purportedly sunk.