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What is the largest successfully or semi-successfully kept secret in human history?

Some definitions:

Successfully - If a secret was kept entirely secret successfully, we wouldn't know about it. On the other hand, if a secret was immediately known by everyone, it wouldn't have been kept successfully. The secret can have been disclosed or discovered prematurely, but should have survived undisclosed for a reasonable amount of time.
Being disclosed or discovered by people not originally "in" on it would not count as being unsuccessful so long as those people were not the ones the secret was being kept from.

Largest - By largest I mean the ratio of the number of people keeping the secret to the number of people the secret was kept from, not (accept perhaps in cases of close size) the significance of the secret.
This is difficult to measure exactly. A secret kept by one person from one other person is usually easily kept and would not be very large. A secret kept by one person from many people would be slightly harder to keep and thus slightly larger. A secret kept by many people from one person would be larger, and a secret kept by many from many, would be even larger. You get the idea.

Some of this may be confusingly worded, so feel free to edit it if you can make it clearer.

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    Well, I could tell you, but then it wouldn't be secret anymore, would it?
    – SJuan76
    Commented Nov 27, 2016 at 23:08
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    I kind of hope we get a moon construction or alien lizard men conspiracy theory on here.
    – AJFaraday
    Commented Nov 27, 2016 at 23:46
  • I think the Manhattan Project might be a good candidate.
    – jamesqf
    Commented Nov 28, 2016 at 5:17
  • I can't see how this is opinion based. It's asking about the largest, not best or anything else.
    – user12566
    Commented Nov 28, 2016 at 7:25
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    @jamesqf the Manhattan project wasn't exactly secret. It was well penetrated by the NKVD (and probably others) from the beginning.
    – jwenting
    Commented Nov 28, 2016 at 9:26

1 Answer 1

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Some candidates, although identifying the single largest secret will be hard:

  • WWII code-breaking secrets: Ultra is the most famous, but the Magic secrets were also very important. Perhaps a hundred people knew all about Ultra, but thousands knew that they were working on code-breaking and kept that quiet for decades after the end of the war.

  • The location of the invasion of north-west Europe in 1944. This was so important that a large deception project, Operation Bodyguard was set up to conceal it. The secret was not kept for a long time, but a large number of people knew it in the weeks before the invasion.

  • The idea of "staged radiation implosion", also known as the Ulam-Teller design for a hydrogen bomb. This was invented in the US in the early 1950s, and kept secret until 1979, although several countries re-invented it.

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