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.
If you’re interested in nuclear weapons, I highly recommend watching the Oscar nominated documentary The Day After Trinity (1981), which you can watch here or on YouTube for free: [Part 1] [Part 2] [Part 3] [Part 4] [Part 5] [Part 6] [Part 7] [Part 8] [Part 9]
The film tells the story of J. Robert Oppenheimer (1904–1967), the theoretical physicist who led the effort to build the first atomic bomb, tested in July 1945 at Trinity site in New Mexico. Featuring candid interviews with several Manhattan Project scientists, as well as newly declassified archival footage, The Day After Trinity was nominated for an Academy Award for Best Documentary Feature of 1980, and received a Peabody Award in 1981.
The film’s title comes from an interview seen near the conclusion of the documentary. Robert Oppenheimer is asked for his thoughts on Sen. Robert Kennedy’s efforts to urge President Lyndon Johnson to initiate talks to stop the spread of nuclear weapons. “It’s 20 years too late,” Oppenheimer replies. After a pause he states, “It should have been done the day after Trinity.” [source]