Eyewitness Report by Luis Alvarez

Morning at 5:30 AM – 16 July 1945

“I was kneeling between the pilot and co-pilot in B-29 No. 384 and observed the explosion through the pilot’s window on the plane’s left side. We were about 20 to 25 miles from the site, and the cloud cover between us and the ground was approximately 7/10. About 30 seconds before the object was detonated, the clouds obscured our vision of the point so that we did not see the initial stages of the ball of fire. I was looking through crossed polaroid glasses directly at the site. My first sensation was intense light covering my whole field of vision. This seemed to last about 1/2 second, after which I noted a fierce orange-red glow through the clouds. Several seconds later, it appeared that a second spherical red ball appeared; still, this apparent phenomenon was probably caused by the airplane’s motion, bringing us to a position where we could see through the cloud directly at the ball of fire, which had been developing for the past few seconds. This fireball seemed to have a rough texture, with irregular black lines dividing the sphere’s surface into many small patches of reddish orange. This thing disappeared a few seconds later, and what seemed to be a third ball of fire appeared again; I am now convinced that this was all the same fireball I saw on two separate occasions through a new hole in the undercast.

When this “third ball” disappeared, the light intensity dropped considerably, and within another 20 seconds or so, the cloud started to push up through the undercast. It first appeared as a parachute blown up by a large electric fan. After the hemispherical cap had emerged through the cloud layer, one could see a cloud of smoke about 1/3 the diameter of the “parachute,” connecting the bottom of the hemisphere with the undercast. This had very much the appearance of a giant mushroom. The hemispherical structure was creased with “longitude lines” running from the pole to the equator. In another minute, the equatorial region had partially caught up with the bars giving a flattened appearance to the top of the structure. In the next few minutes, the system’s symmetry was broken up by wind currents at various altitudes, so the cloud’s shape cannot be described in any geometrical manner. In about 8 minutes, the top of the cloud was at approximately 40,000 feet, as close as I could estimate from our altitude of 24,000 feet, which seemed to be the maximum altitude attained by the cloud. I did not feel the shock wave hit the plane, but the pilot felt the reaction on the rudder through the rudder pedals. Some of the other passengers on the aircraft noted a relatively small shock at the time, but it was not apparent to me…”

Luis W. Alvarez

Source: U.S. National Archives, Record Group 227, OSRD-S1 Committee, Box 82, folder 6, “Trinity.” Transcription: Thank you, Gene Dannen for transcribing this document.