What is the "Crack of Doom"?

[John Middlesworth <jape@email.unc.edu>]

I had the impression that the MGB "crack of doom" was a crack that
revealed the unibody was falling apart.  Recently though, I've heard it
described as the crack that might run from the outside rear-view mirror to
the top of the door.  If it's the latter, why is there "doom"?  Couldn't 
the crack simply be welded?

John Middlesworth
1972 MGB