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You were getting very close indeed! Here's the answer, which I found in 'U-boats' by David Miller:
The reported "explosion" sound was investigated in tests carried out during 1947 by the Royal Navy. Unmanned surplus submarines were lowered into the sea until they collapsed.
Previously, it was thought that a submarine's hull gave way gradually at its critical crush depth.
The tests showed that in fact the hull suffered a sudden, catastrophic implosion - the unexpected "exploding" sound heard, in the absence of a corresponding depth charge, by many ASW ships as their opponents finally met their violent end.
The book doesn't mention whether or not these implosions were visible on the surface. I would expect that the huge force and size of an entire U-boat imploding in an instant would result in a large shower of water on the surface, similar to that of a depth charge explosion.
No-one quite provided the full answer - but wh1skea partially got it right, and Messervy mentioned the all-important word "implosion". So do you want to decide between yourselves? Or maybe whoever posts a question first?
Otherwise, I have another question lined up
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