I had thought it might be the Port Chicago explosion, but there are some other very large accidental detonations during the war, including one in Bergen, Norway that sounds larger.

The [Grand Slam](https://en.wikipedia.org/wiki/Grand_Slam_(bomb)) was the biggest bomb of the war, aside from Fat Man/Little Boy. Ive always heard that the Port Chicago accident was a lot larger than any other disaster. I havnt heard about the Bergen detonation that Restricteddata suggested.