Just a thought experiment, let’s say the earth is flat, looks like many professionals would know, including
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Pilots/ATC/sea captain
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Land surveyor and civil engineers
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Astronomers and Geologists, at least the one dealing with experimental data
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Militaries, at least radar operator and artillery
Just these categories are already a lot of persons, I assume I miss a lot of others. So how many person does it make in total ? 10 % of the population ?
Manhattan leaked before they even started work.
From memory, a sci-fi magazine seller figured it out, just based on address changes. He even figured out it was likely an attempt to make an atomic bomb. The USSR also had it completely compromised. The first soviet bomb was basically a direct copy of the American one.
That was also with most/all the workers in 1 location. They also had a very good reason to maintain security (grunts on the ground, not just leadership).
Wasn’t the Manhattan project compromised from within because of communist sympathizers, or even outright communists working on the project?
Correct, though that wasn’t the only leak, just the most serious.