Meredith Gardner broke a Soviet code that revealed Soviet espionage in the Manhattan Project, and Soviet activity in the Treasury Dept, the State Dept and even the White House. The code breaking, a collaboration between the US and the UK to break Soviet coded messages, was known as the VENONA Project. It was so secret that Pres. Roosevelt and Truman knew nothing about it and led to evidence used in such spy cases as Julius and Ethel Rosenberg.
While Mr. Gardner was the key man in breaking this code, most of the code breakers were young women.