alt Hacker NewsMy own guess is that a fair number of them just buried themselves. While I don't have all the relevant facts at hand, per the US Naval Institute, a 550 lb bomb dropped form 20,000 feet would hit the ground at 880 feet per second [1]. If my rusty mind is correct, that would be around 9 megajoules, while feels like it ought to be enough to bury a bomb pretty well in any sort of yielding soil.
1. https://www.usni.org/magazines/proceedings/1941/december/how...
Have another bomb in the vicinity that did not fail and there will be absolutely no visible indication at all that something penetrated the ground.
I might be imagining this but I think that I have seen a report about some systematic effort to trace bomb dropping patterns in aerial images of the aftermath to find gaps in the pattern where something did not go off. But no idea how early or late after the war that happened (if it did happen). And those patterns will only show up for smaller bombs dropped in quantity, not for the big ones.