The Trinity test was the first test of a nuclear weapon by the United States. It took place on July 16, 1945. It was the first explosion in the history of a weapon of this type. The detonated bomb used plutonium fissile material, like the one released later on Nagasaki, Japan.
At the time of the detonation, the surrounding mountains were illuminated for one to two seconds. The observed colors of the illumination varied from purple to green, and finally to white. The explosion took 40 seconds to reach the observers and the shock wave could be felt 160 kilometers away. The mushroom cloud reached 12 kilometers.
The Army reported the event as an accidental explosion in an area of ammunition disposal, which was not denied or made public until August 6, after the attack on Hiroshima.