In the early 1950s, the U.S. and Russia got into a race to develop the first aircraft-deliverable nuclear bomb. But the Americans accidentally created a much more powerful bomb than they anticipated. What they thought would be a 5-megaton nuclear explosion generated a 14.8-megaton blast.
The Castle Bravo test at the Bikini Atoll in 1954 was the first dry hydrogen explosion that the U.S. attempted and it used lithium deuteride as the fusion fuel. But lithium deuteride is much stronger than the scientists thought.
So the Americans set up the islands and the safe zones for an explosion of 5-6 megatons. The immediate area was evacuated, they checked the wind speeds to limit the spread of contamination, and they positioned all of their facilities in safe areas.
But the 14.8-megaton explosion in Castle Bravo rendered many of these preparations moot. The small strip of land that the device was tested on was wiped out and became a crater 6,510 feet wide and 250 feet deep.
All the soil that had been an atoll flew into the atmosphere along with disintegrated coral reef. These later fell as a powdery ash on unsuspecting Japanese fishers and Pacific Islanders.
One of the Japanese fishermen soon died of acute radiation poisoning while the rest of the victims affected suffered dramatically increased rates of cancer and other diseases.
Despite the costs, the Castle Bravo test did lengthen America's lead of the nuclear arms race, but it didn't keep the top spot for nuclear explosions.
The largest ever nuclear explosion was Russia's Tsar Bomba, a 50-megaton device that was tested in 1961.