Trinity was the code name of the first detonation of a nuclear device. This test was conducted by the United States Army on July 16, 1945. The date of the test is usually considered to be the beginning of the Atomic Age.
The above picture is the Trinity explosion, 0.016 seconds after detonation. The fireball is about 600 feet (200 m) wide. The black specks silhouetted along the horizon are trees.