In 1946, the United States conducted a series of nuclear weapon tests at Bikini Atoll in what’s known as Operation Crossroads. A total of two bombs were detonated to test the effects nuclear blasts had on naval warships. The second, named Baker, was the world’s first nuke to be detonated underwater. Due to the unique properties of underwater explosions, the Baker test produced a number of unique photographs that the world had never seen before.
There were many issues with cleaning up after the test, the life expectancy of people involved was reduced by an average of three months, and the test was later referred to as, “the world’s first nuclear disaster.” You can read up on the whole operation in detail on its Wikipedia article