World's First Underwater Nuclear Explosion

In 1946, A series of nuclear weapon tests, called Operation Crossroads, conducted by the United States in Micronesian island of Bikini Atoll. A total of two bombs were detonated to test the effects of nuclear weapons on naval warships. The U.S. blasted a 23-kiloton nuke known as “Baker” 90 feet underwater. The explosion caused ocean water to reach over two miles high. Nuclear tests continued until 1958. 

Crossroads Baker, showing the white surface "crack" under the ships, and the top of the hollow spray column protruding through the hemispherical Wilson cloud.

Mushroom-shaped cloud and water column from the underwater Baker nuclear explosion of July 25, 1946.

The Wilson cloud has evaporated revealing the cauliflower atop the spray column. Two million tons of water spray fall back into the lagoon. The radioactive base surge is moving toward the ships.