Eyewitness Report by Kenneth Greisen

July 21, 1945

To: Lt. Taylor
From: K. Greisen

“A group of us were lying on the ground just outside base camp (10 miles from the charge) and received time signals over the radio, warning us when the shot would occur. I was personally nervous, for my group had prepared and installed the detonators, and if the picture turned out to be a dud, it might be our fault. We were sure we had done our job well, but there was always some chance of a slip.

At minus about 15 seconds, I put my head close to the ground, looked away from the tower, and put up a shield between my head and the building. I probably also closed my eyes briefly just before the shot. Suddenly I felt heat on the side of my head toward the tower, opened my eyes, and saw a brilliant yellow-white light all around. The heat and light were like the sun had just come out with unusual brilliance. About a second later, I turned to look at the tower through the dark welding glass. A massive cloud of smoke was pouring upwards, some parts having brilliant red and yellow colors, like clouds at sunset. These parts kept folding over and over like dough in a mixing bowl. At this time, I believe I exclaimed, “My god, it worked!” and felt a great relief.

When the intensity of the light had diminished, I put away the glass and looked toward the tower directly. At about this time, I noticed a blue color surrounding the smoke cloud. Then someone shouted that we should observe the shock wave traveling along the ground. The appearance of this was a brightly lighted circular area near the bottom, slowly spreading out towards us. The color was yellow.

At what I presume was about 50 seconds after the shot, the ground shock and sound reached us almost simultaneously. The noise lasted long, echoing back and forth from the hills. I noticed no sharp crack but a rumbling sound like thunder. After our brilliant optical display, the ground shock and noise were disappointing. No damage occurred, and we were not at all severely shaken.

Between the appearance of light and the arrival of the sound, there was loud cheering in the group around us. After the noise was over, we all congratulated each other and shook hands. I believe we were all much more shaken up by the shot mentally than physically.

The permanence of the smoke cloud was one thing that surprised me. After the first rapid explosion, the lower part of the cloud seemed to assume a fixed shape and remained motionless in the air. Meanwhile, The upper part continued to rise, so it was at least five miles high after a few minutes. It slowly assumed a zigzag shape because of the changing wind velocity at different altitudes. The smoke had pierced a cloud early in its ascent and seemed utterly unaffected by the cloud.”

K. Greisen

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.