One Park, Three Sites, Countless Stories The Manhattan Project is one of the most transformative events of the 20th century. It ushered in the nuclear age with the development of the world's first atomic bombs. The building of atomic weapons began in 1942 in three secret communities across the nation. As World War II waned in 1945, the United States dropped the atomic bombs on Hiroshima and Nagasaki, Japan - forever changing the world.