Originally Posted by JohnDThompson

That's a bulge-formed bottom bracket shell. Those often do not have direct tube/shell contact for the full depth of the tube socket, leaving a stress riser that can crack. I've repaired such a failure by cutting the shell into pieces, sweating off the pieces, and installing a new investment-cast shell. It's a big job, and not likely worth the expense unless you're doing the work yourself.

N.B. still riding the repaired frame 40+ years later.