This is a typewritten account of the detonation of "Trinity," the first atomic device, on July 16, 1945, written by physicist Luis W. Alvarez, who witnessed it from a B-29 aircraft flying 24,000 feet over the site. In this report, Alvarez comments upon the detonation itself, the formation of the mushroom cloud, and the observations of the shock wave by the pilot and other passengers in the aircraft.