A nuclear test detonated as part of the Operation Castle series of atmospheric tests performed by the United States between 1945 and 1963, when the Nuclear Test Ban Treaty was signed. It is the highest-yield nuclear detonation ever carried out by the U.S.
Operation Castle tested high-yield thermonuclear devices, which were advancements of the device detonated in the Ivy Mike test. One of these advancements was the use of "dry" fuel that didn't need to be cryogenically frozen - in this case, lithium deuteride. In fact, the USSR detonated a device using this fuel the August prior. This and other developments produced yields dramatically exceeding expectations; indeed, the three largest nuclear explosions ever carried out by the United States were all part of Operation Castle.
Castle Bravo took place on March 1st 1954, in the Pacific Proving Ground. The test employed a "Shrimp" device - a shorter version of the "Runt" device used in the Castle Romeo test. It weighed 23,500lb and was just under 15ft long. The device was detonated in a surface burst and produced a yield of roughly 15 megatons, about two and a half times higher than the most optimistic prediction of 6 Mt.
The explosion blew a hole 6510ft wide and 250ft deep out of Bikini Atoll. After a minute the mushroom cloud had reached 50,000ft (15 kilometres) and in another two minutes had passed 100,000ft, eventually topping out at 130,000ft (40km) after six minutes. At various stages in its development the cloud displayed four condensation rings, two skirts and three ice caps. At full size, eight minutes after detonation, the cloud was 100km across with a 7km stem. This engulfed most of the diagnostic pipe array, all the way up the earth-banked instrument bunker which barely survived.
The unexpectedly high yield was down to the lithium-7 isotope present in the fuel. This isotope was expected to be essentially inert, but in actual fact reacted very strongly to the high-energy neutrons produced by the fusion of tritium and deuterium. It caused the isotope to fragment into tritium and helium atoms; tritium is a very valuable fusion fuel as it is highly reactive and provokes very energetic fusion.
This test produced the worst radiological disaster in US history. The inhabited atolls of Rongelap, Rongerik, Ailinginae and Utirik were all contaminated with radioactive material due to changes in the weather prior to the test. A Japanese fishing vessel, the "Daigo Fukuryu" ("Lucky Dragon") was contaminated with the same "gritty white ash" (actually calcium precipitated from vaporized coral) that fell on the Marshall Islands in the fallout plume.
Given that subsequent testing would see the Pacific Proving Ground rapidly running out of islands, the next test (Castle Romeo) was conducted from a barge over the crater left behind by the Castle Bravo test.
Source: <http://nuketesting.enviroweb.org> (now offline)