'Castle Bravo' on 1 March 1954, Bikini Atoll:
Largest U.S. test in terms of yield and fallout.

1 March 1954 - Castle Bravo

The crater left by the Castle Bravo test is visible from space.

The first of March 1954 marks one of the most serious nuclear fallout incidents in history. On this day, the United States conducted its largest ever nuclear weapon test, code-named Castle Bravo, at the Bikini Atoll in the Marshall Islands. Bravo was part of Operation Castle, a nuclear test series designed to develop an aircraft-deliverable thermonuclear weapon. Due to a design error, the explosion reached a yield of 15 megatons, making it two and a half times larger than expected and more than 1,000 times as powerful as the Hiroshima bomb. The largest-ever nuclear explosion was the 1961 Tsar Bomba with 50 megatons.

Fallout pattern of the Bravo test. Source: atomicarchive.com.

Radioactive fallout from the test spread over more than 11,000 square kilometres. Traces of radioactive material were detected in Australia, India, Japan, the United States and Europe. The Bikini population had been relocated to other atolls prior to the start of the U.S. nuclear testing programme in the Pacific with the Able test in 1946. Due to the unfavourable weather conditions in which the Bravo test had been conducted, the fallout also affected the inhabited atolls of Rongelap, Utrik and others.

A yellow flash poured through the porthole. Wondering what had happened, I jumped up from the bunk near the door, ran out on deck, and was astonished. Bridge, sky, and sea burst into view, painted in flaming sunset colors. I looked around in a daze; I was totally at a loss.Oishi Matashichi, fisherman aboard The Lucky Dragon #5
The luckless ‘Lucky Dragon’

In addition, radioactive fallout heavily contaminated the Japanese fishing vessel “Lucky Dragon #5”, which was sailing some 145 kilometres downwind from ground zero. The 23 Japanese fishermen aboard suffered from radiation poisoning and one crew member died shortly after. The incident resulted in a diplomatic crisis between Japan and the United States and gave rise to international criticism of nuclear testing. A few weeks after the test, Indian Prime Minister Jawaharlal Nehru advocated a “standstill agreement” on nuclear testing (see chapter Early Efforts to Restrain Nuclear Testing).

Approximately five hours after detonation, it began to rain radioactive fallout at Rongelap. Within hours, the atoll was covered with a fine, white, powdered-like substance. No one knew it was radioactive fallout. (…) The children played in the snow. They ate it.Senator Jeton Anjain on behalf of the Rongelap Atoll Local Government
Ōishi Matashichi, a fisherman aboard The Lucky Dragon #5, tells the story of the Bravo test.

U.S. testing in the Marshall Islands has had long-term effects on health and the environment. A medical report (PDF) compiled by the National Institute of Health, Maryland, and the Brookhaven National Laboratory, New York, found that the development of thyroid diseases has been the major late effect of radiation exposure of the Marshallese people. In 1972, the U.S. Atomic Energy Commission declared (PDF) the islands had seen a “remarkable recovery” and more than 100 people moved back to Bikini. However, laboratory tests in 1978 indicated that unacceptably high levels of radiation remained on the atoll and Bikini was evacuated again. In 1997, the International Atomic Energy Agency (IAEA) concluded that Bikini “should not be permanently resettled under the present radiological conditions.”

During the second and third decades after the accident, most of the Rongelap children and many adults developed thyroid nodules, some of which proved to be malignant.National Institute of Health, Maryland, and Brookhaven National Lab, NY
Islanders had been asked to "temporarily" leave Bikini "for the good of mankind and to end all world wars" (U.S. Archives)

In 1964, the U.S. government appropriated funds to compensate Marshallese people who were exposed to fallout from its testing programme and in 1988 the Marshall Islands Nuclear Claims Tribunal was established to represent the interests of those affected. Claiming the compensation schemes to be inadequate, Bikini Islanders have taken legal action. In 2010, the U.S. Supreme Court declined to hear any new cases in pursuit of more compensation.

[W]e are sadly more akin to the Children of Israel when they left Egypt and wandered through the desert for 40 years. We left Bikini and have wandered through the ocean for 32 years and we will never return to our Promised Land.Bikinian representative Tomaki

The United States carried out 67 nuclear weapon tests in the Pacific Ocean between 1946 and 1958 and a total of 1,032 nuclear tests overall. In 1996, the United States was the first country to sign the Comprehensive Nuclear-Test-Ban Treaty (CTBT) that bans all kinds of nuclear explosions, everywhere. However, it has yet to ratify the Treaty, a step that is mandatory for the CTBT to become global law.

Declassified U.S. nuclear test film on the Operation Castle: