One argument against large scale conspiracies being true

"The argument is often advanced that the non-existence of any given conspiracy is shown by the lack of leakers or whistle blowers. Given the success of the British government in getting thousands of people to keep the ULTRA secret -- and thereby ensuring that no reliable history of World War II could be published until the 1970s -- it is apparent that this is not necessarily a reliable indicator."

from Wikipedia - "conspiracy theory/real conspiracies"

Often the argument is advanced that a conspiracy on a grand scale is not possible because of a supposedly large number of people who would have to be in on the secret, and surely someone would "out" the conspiracy. It's been proven in the past that this is not necessarily true.

The best known example I can think of is the WWII "Manhattan Project", where thousands of people were working on developing the atomic bomb, but only a very small inner circle knew exactly what was being worked on. Everyone worked on their own little piece of the project, and was told only what they needed to know to complete their own task, and no more. Of course even this was kept secret as even small snippets of information could be valuable to the enemy, but it was possible to pull off a large project involving thousands of technicians without the cover being blown, at least to the general public anyway..... The Soviets found out about it because of their mole spy Klaus Fuchs, and others within our own government, but the general public was successfully kept in the dark about it.