Biggest successful secret project in history?

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.

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.

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.

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.

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:
  • Bismarck fully loaded displaced 50,300 tons.
  • Battleship Iowa, fully loaded displaces 57,500 tons.

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.