slashc

There's actually quite a few lost nuclear bombs just lying around on the ocean floor http://www.genecurtis.com/LostNuclearBombs.htm

there's several at the top of this page

It was probably a WB-57F or similar recon variant which had a longer wingspan so it could fly at higher altitudes. A nuclear mushroom cloud can reach up almost 50,000ft which would probably be near the maximum of a regular B-57, but the versions with longer wings can go much higher. NASA actually stills uses a couple of modified long wingspan B-57s with newer engines even today.