Hey r/AskHistorians, I remember a year or two ago I heard an anecdote about the Manhattan project, specifically about how initial estimates produced fears that lighting off the bomb could light the Earth's atmosphere on fire and kill everyone. As I remember it being told, someone brought these concerns and the math involved to Einstein, who told them to check it again and then a third time, and if the math checked out all three times, he'd do whatever he had to to bury the project, saying something to the effect of living under Nazi rule would be better than literally everyone on Earth dying.
However, for the life of me I can't relocate wherever it was I heard this from, and despite multiple searches I've never been able to find any sources for this, so I'm dreadfully curious if this is true or not.
Either what you were told or your memory is a little confused. Einstein wasn't part of the Manhattan Project. The kind of physics he does is not really germane to the practical production of nuclear weapons, despite popular perception. He was also considered too politically suspicious. So Einstein wasn't really part of anything, except for signing a letter to FDR about fission research in 1939.
There were some brief thoughts that the bomb might ignite a fusion reaction in the atmosphere (nothing to do with lightning). This was fairly early in the project. The same people who thought this idea up also figured out that it wouldn't happen. The idea was floated to a few people higher up in the project, and one of them, Arthur Compton, was indeed of the opinion that if there was any plausible chance of that happening, then they shouldn't proceed. But there wasn't any real plausible chance of it happening.