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Many conspiracy theories rely on a powerful organisation (government agencies, secret societies, etc.) hiding the truth from public view. A prime example are alien conspiracies.

To the known record, what is the biggest secret project that was successfully pulled off in history?

The answer should satisfy the following criteria:

  1. The size of project should be measured in number of people actively working on the project (either aware or unaware).
  2. The project should have been successfully kept secret for a long period of time. For the sake of concreteness, let's say at least 5 years.
  3. Bonus points if the project is international.

(Perfect secrecy, that is nobody outside the project knowing anything about the existence of the project, is basically impossible, so projects in which very few outsiders knew about it are still considered fine.)

The answer to this question is relevant to assess the likelihood of a given conspiracy theory.

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    I have to give a frame-challenge. The BEST secret projects are the ones that are STILL secret. By definition we don't know about them (yet). So, whatever answers are given, there is no telling for sure if they are actually the biggest projects. – Tonny May 1 at 9:00
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    I'd say we probably don't know about it – user2723984 May 1 at 9:10
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    The Magathreans (or rather, Earth as a whole) probably came oh-so-close to holding the record. – chepner May 1 at 13:06
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    I'd tell you, but then I'd have to kill you. – Mark May 1 at 21:24
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Question: Biggest successful secret project in history?

I would have to go with the Manhattan Project.

Manhattan Project? It employed 125,000 people at its peak, and 610,000 people cumulatively. It was active from 1939-1947, but only secret for six of those years (August 6, 1945). And yes, I'm going after the bonus points. It was an international project. There were Canadian sites in British Columbia, Ontario, and the Northwest Territories. Additionally many UK and Canadian scientists worked on the project as there was an agreement to work jointly and share the technology. British contribution to the Manhattan Project. Not to mention a few clandestine Soviets.

Manhattan Project A 1945 Life article estimated that before the Hiroshima and Nagasaki bombings "probably no more than a few dozen men in the entire country knew the full meaning of the Manhattan Project, and perhaps only a thousand others even were aware that work on atoms was involved."

Honorable Mention:

  • Japanese Battleship Yamato - It was the largest battleship of WWII, It's 70,000 tons of steel impacted total Japanese steel imports. Design began in 34, construction began in 37, commissioned the week after Pearl Harbor, and became operational in 42. Western Intelligence got it's first look at the Yamato in February 1944, reconnaissance photographs, and still underestimated its size by 10,000 tons. For Scale:

Yamato was a huge undertaking, and a closely held secret for nearly 8 years. Not international and I could find no numbers on Labor.

Comments:

@Barry - Although the project was successful, it was far from secret. The Russians knew about it well before the bomb was dropped on Hiroshima thanks to spies such as Klaus Fuchs and the Rosenbergs.

The Russians were allies during WWII, and were told about the bomb prior to its use. President Truman informed Stalin about the existence of the bomb at the Potsdam Conference, July 24, 1945. It's true that Stalin knew about the existence of the bomb before Truman told him, but given Truman informed Stalin prior to it's use, suggests the "secret" was more about Japan and Germany than US allies. The level of secrecy of the Manhattan project was exceptional. Vice President Truman wasn't told until 13 days after he became President.

Truman and the Atom Bomb
When Truman became president on April 12, 1945, upon the death of President Roosevelt, he had no knowledge of the actual bomb project itself and his first information about what was really being done came from Secretary of War Henry L. Stimson on April 25th.

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    Depending on how you define "international", several international (refugee) scientists were involved, not only from the UK, Canada, but also from axis countries like Italy, Hungary. – Greg Apr 30 at 19:41
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    Although the project was successful, it was far from secret. The Russians knew about it well before the bomb was dropped on Hiroshima thanks to spies such as Klaus Fuchs and the Rosenbergs. – Barry Apr 30 at 22:57
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    @Barry It is hard to tell an absolute scale for secrecy. You can call it a secret when your enemies have no faintest idea about it, neither majority of the people who work there. – Greg May 1 at 2:22
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    The project to design and build the B-29 Super Fortress bomber was bigger than the Manhattan Project. "The $3 billion cost of design and production (equivalent to $43 billion today)—far exceeding the $1.9 billion cost of the Manhattan Project—made the B-29 program the most expensive of the war" – Pieter Geerkens May 1 at 6:57
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    What about the D-Day landings? Not long, but think about the scale of the planning, preparation, and the initiation of it. Surely that required tons of people, no? The fact that a invasion of France was going to happen wasn't a secret, but the location of it sure was. – Rewan Demontay May 2 at 1:01
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I nominate the breaking of the code generated by the German Enigma machine during World War II by British cryptographers. Hundreds, if not thousands, of people were involved with the project which was ongoing throughout the war. The secret was not revealed until many years after the war ended. The Germans never knew that their codes were broken.

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    Definitely this. 10,000 people worked in the Bletchley Park organisation as a whole. The secret was kept for 40 years. You could improve your answer by adding these facts. – Graham May 1 at 8:26
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    Sources would improve this answer. Also, the numbers don't seem to even come close to the (sourced) numbers given by JMS for the Manhattan project. – Lars Bosteen May 1 at 8:58
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    @LarsBosteen, it would have a longer secrecy period than the Manhattan project. And one could add the American side, Magic/Ultra. – o.m. May 1 at 14:26
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    @LarsBosteen, in terms of number of person-years of secrecy, they're comparable. The Manhattan Project was bigger, but breaking Enigma remained secret for 7-10 times longer. – Mark May 1 at 21:33
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We can't know the biggest successful secret project, just big revealed ones. Here are some examples with 5, 10, 20 years duration.

  • Project Azorian ran for about 5 years from start to disclosure. 160 on the ship, countless more on shore.
  • The F-117. Contracted in 1978, except that the 1xx designator fits into a much earlier era. The existence was officially denied until 1988, even if there were rumours.
  • DKP-MO. 200 west German communists trained for guerilla war from 1969-1989.
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    Depending on how you interpret the "success" part of the question, you could add (Operation Gladio](en.wikipedia.org/wiki/Operation_Gladio) to the list. It was not "successful" in that it was not "activated", but it remained secret and certainly it was way bigger than DKP-MO. – SJuan76 May 1 at 11:14
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    Where would Stuxnet fit into this? Also, speaking of airplanes, how about the SR-71? It was operational in the sixties but I think it was pretty hush-hush for quite a long time. – CramerTV May 1 at 20:33
  • @CramerTV the SR-71 was public before it ever flew - you might be thinking of its predecessor, the A-12 and its variants, which flew before the SR-71 was developed. While similar in shape, they are not the same aircraft. – Moo May 2 at 11:45
  • Project Azorian developed a very robust facade, kicking off research in the field of deep sea mining. Very impressive. – Dohn Joe May 3 at 16:22
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From 1950 to 1960, "Psychic driving" was a CIA program related to MKUltra which involved medical tests erasing people's minds using drugs and electricity and reprogramming them with tape recordings played thousands of times. https://en.wikipedia.org/wiki/Psychic_driving

Operation Legacy ran from the 50's to the 70's and it involved erasing all of the embassies files from all the UK colonies, erasing all records of the administrative side of military colonialism and suppression of rebellions.

The NSA and GCHQ run complex groups of surveillance technologies that can collect billions of records in a few days, there are many of them, for example BLARNEY which ran from 1978 to 2013 to collect telecoms records: https://en.wikipedia.org/wiki/Blarney_(code_name) https://en.wikipedia.org/wiki/MUSCULAR_(surveillance_program)

The Chinese spent 17 years building the biggest artificial underground cave system for a nuclear program: The total length of the tunnels is more than 20 kilometers in 13 levels, including 18 artificial caves, it has more than 80 roads and 130 tunnels. The largest artificial cave has a height of 79.6 meters, and is roughly equal to that of a 20-floor building. https://en.wikipedia.org/wiki/816_Nuclear_Military_Plant

Pakistan developed ballistic missiles from 1988 to the mid 1990's and the world was surprised to learn that India and Pakistan were both testing nuclear bombs in 1998.

The USA has ran many secret aircraft programs that spanned decades and cost billions of dollars like the blackbird and stealth bombers(the stealth cost 770 million per aeroplane), the same as 10 boeing 747's.

The holocaust is also worth mentioning, because it was a major secret government program.

There are thousands of secret government programs that lasted more than 5 years with unbelievable aims, trick is knowing the most consequential for cost and effect. Snowden revealed dozens of programs tasked with bugging every possible telecoms equipment in existence from google's data center cables to every kind of phone company.

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A very large successful secret project was the violation/revision of the Versailles treaty by Germany, over the 1920s and early 1930s.

It began no later than about 1922, with a German-Russian "understanding", following the formal Treaty of Rapallo. At that time, Germany tested tactics using such "forbidden" weapons as tanks and airplanes on Soviet soil, sharing the results with the Soviet Union. This was an "international" part of the violation.

Up to about 1930, the German Navy built a number of ships that were technically within the limits of the Versailles Treaty, but more heavily armed than the Allies intended. In the early 1930s, the Germans extended this to building ships that were bigger than allowed. In 1935, Germany concluded a separate Naval Treaty with Britain that allowed Germany to exceed the Versailles limits, and placed a new, higher cap on the German navy at 35% of Britain's.

The last step involved about half a million people, when Germany increased her army in 1935 from the Treaty-limited 100,000 men to 550,000. Counting all the previous violations, perhaps going on 1 million people had been involved, including Russians and other non-Germans.

Early in 1936, German Army units marched into the Rhineland. By this time, the violation of the Versailles Treaty was no longer secret. Starting in the summer of 1936, several hundred men of Germany's Condor Legion provided air support to Franco's Nationalists in Spain. This support itself was no secret, but the earlier recruitment, training, and supplying of the Legion, which involved thousands of people, had been secret.

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