The first of the two nuclear bombs used against Japan was a uranium bomb dropped on the city of Hiroshima. The second bomb, which destroyed Nagasaki, was a plutonium bomb. U.S. authorities soon realized that plutonium bombs weighed less but had a bigger explosion, so they decided that all future U.S. nuclear bombs would be made with plutonium. The Austin Company of Cleveland, Ohio, was given the task of selecting a site for this plutonium work. They chose a place called Rocky Flats, 16 miles northwest of downtown Denver.