25 July - Operation Crossroads in 1946

On this day - 25 July 1946 – Operation Crossroads: an atomic bomb is detonated underwater in the lagoon of Bikini Atoll. 

Operation Crossroads was a pair of nuclear weapon tests conducted by the United States at Bikini Atoll in mid-1946. They were the first nuclear weapon tests since Trinity in July 1945, and the first detonations of nuclear devices since the atomic bombing of Nagasaki on August 9, 1945. The purpose of the tests was to investigate the effect of nuclear weapons on warships.

The Crossroads tests were the first of many nuclear tests held in the Marshall Islands, and the first to be publicly announced beforehand and observed by an invited audience, including a large press corps. They were conducted by Joint Army/Navy Task Force One, headed by Vice Admiral William H. P. Blandy rather than by the Manhattan Project, which had developed nuclear weapons during World War II. A fleet of 95 target ships was assembled in Bikini Lagoon and hit with two detonations of Fat Man plutonium implosion-type nuclear weapons of the kind dropped on Nagasaki, each with a yield of 23 kilotons of TNT

The modern bikini was invented by French engineer Louis Réard in 1946. He named it after Bikini Atoll in the Pacific, the site of an atomic bomb test on July 1, 1946. Réard hoped that the burst of excitement it caused would be as explosive as an atomic bomb.