1954 castle bravo
On March 1st, 1954, the United States conducted its largest nuclear weapons test near the Marshall Islands, releasing radiation into the atmosphere. Because of a design error, the explosion was two and a half times larger than previously anticipated. The explosion reached about 15 megatons, and was 1000 times stronger than the Hiroshima bomb. Traces of radiation were detected everywhere from Australia to Japan to the United States. Unfortunately, an unlucky fishing vessel called the "Lucky Dragon #5" got caught just 145 kilometers from the explosion. All members aboard the ship ended up suffering from radiation poisoning, and one member even died. In 1978, tests revealed that the radiation in the Bikini Atoll area was still at an unacceptably high level. It was concluded that the area should not be permanently resettled.
"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, a fisherman aboard the Lucky Dragon #5
Residents of Bikini Atoll in the Marshall Islands were told to "temporarily" leave the area.
Picture Source: http://www.ctbto.org/specials/testing-times/1-march-1954-castle-bravo/
The crater left from the explosion, as seen from space.
Picture source: http://www.ctbto.org/specials/testing-times/1-march-1954-castle-bravo/