The Manhattan project was such a gargantuan effort -- I'm surprised it was kept so well under wraps from whistle blowers. Or was there some level of public awareness that was actively squashed and/or dismissed by the government, media, or the public at large?
I understand that the USSR actually did infiltrate the organization, but that was government to government espionage rather than whistle blowing or internal exposure to the media.
This question kind of popped into my head considering all the recent leaks i.e. Snowden, Manning, Assange...
The Manhattan Project used a lot of different techniques to try and maintain its secrecy. These included, but are not limited to:
Impressing onto the scientists just how in trouble they would get if they broke secrecy, including making them swear an oath than their scientific reputation would be besmirched, along with going to jail and stuff
Compartmentalizing the work so that the vast majority of the people working on the project had no idea what they were actually building
Locating the most secret parts of the project in "secret cities" that were basically military colonies, constantly monitored by counter-intelligence and military police, behind barbed-wire fences
Occasionally giving out misinformation to throw people off the trail
Occasionally invoking the voluntary censorship codes to suppress newspaper, magazine, and radio media
Occasionally using intimidation (and maybe worse) to get troublesome people to stay quiet
Getting half-a-dozen top Congressmen on board to make sure none of the others questioned the expenditures
Occasionally making appeals to patriotism and/or fear to get labor union leaders on board, and other troublesome but important people
...among other methods.
Now, before you are too impressed, the question of "did it work?" is actually hard to parse out. There is the obvious problem of the Soviet spies, which had no real difficulty infiltrating.
What about the media? What about Congressmen? Here it is a much more spotty record. There were dozens and dozens of leaks in the media. Some of them minor, some of them major. Here is the one I have somewhat arbitrarily dubbed the "worst" of the leaks, though there are others that are pretty bad. This one identifies the secret Los Alamos lab as a target, identifies new explosives as what is being produced, identifies the top scientist on the project (Oppenheimer), and indicates the degree of secrecy that surrounds the site. If you were a member of German or Japanese intelligence reading that article, you'd say "WTF, that is worth looking into more."
There were plenty of other leaks, some just as bad. It was an "open secret" among some circles in Washington that there was a big project named Manhattan that was making some kind of secret weapon. There were some attempts by whistleblowers of a sort to get in touch with Congressmen, the President, and so on — they were rebuffed and threatened and generally quieted.
My general take on it is that if the Japanese or Germans — or anyone else — had been looking for an atomic bomb program, they would have found it. It was easy to see if you knew what to look for and were looking. They were very lucky it was as secret as it was, and it was leaked far more than anybody really appreciates.
The Manhattan Project security people knew exactly how shaky their secrecy was and knew it could be blown at any minute. They actually made some preparations as to what to say if they were forced to reveal/confirm the existence of the program before using the bomb. And they made a lot of preparations for how to talk about the bomb after it was used — because they understood that in that first rush of publicity a lot of bad information could get out.
So the lesson I would take away from it is that if you have a secret that requires the labor of 500,000 people or so to produce, even if only 1% of those people know the secret, it is going to be a very hard one to keep and a very imperfect thing to control, even in 1945, much less today, where information is more readily transmitted and coordinated. The Manhattan Project's major secret — that there was a project to build an atomic bomb — was an extremely tenuously kept, extremely temporary secret. It is not the sort of thing that you would want to keep as a long-term secret, because it is too large. The Manhattan Project was basically the creation of an entire industry from scratch, and that is a very hard thing to hide (much harder than, say, any particular detail or process in the making of the bomb — postwar secrecy was a very different sort of affair than wartime secrecy).