What was the Manhattan Project?

The project's roots lay in scientists' fears since the 1930s that Nazi Germany was also investigating such weapons of its own. Born out of a small research program which began in 1939, the Manhattan Project would eventually employ over 130,000 people and cost a total of nearly $2 billion USD ($20 billion in 2004 dollars based on CPI), and result in the creation of multiple production and research sites operated in secret.