In 1946, the US government started conducting tests of nuclear explosions at Bikini Atoll, Marshall Islands.  The tests began with Operation Crossroads, a series of 2 atomic tests codenamed Able and Baker.   Although both bombs were detonated to measure effects of nuclear weapons on warships, Able was an atmospheric test, resulting in Baker being the first nuclear detonation underwater. 

Detonated 90ft below the water, the 23-kiloton bomb created a column 6,000ft tall and 2,000ft wide, lifting some 2 million tons of sand & water (and a few ships, one visible at right side of column base) skyward.  Also created? Contaminated sea spray and a radioactive mist, resulting in large-scale contamination of the entire target fleet, lagoon, surrounding environment and subsequent termination of Operation Crossroads.

Photo dated July 25, 1946, taken from a distance of 3.5 miles.