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:
- The size of project should be measured in number of people actively working on the project (either aware or unaware).
- 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.
- 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.