This nuclear blast went off in 1946 at Bikini Atoll in Micronesia. How did some of the radiation get back to the United States? Why, we imported it, of course!
Has the radiation from nuclear testing abroad come back to haunt the United State via ocean currents and wind patterns? Probably. But we found a more direct way of getting it back home. If you look at the picture above, you'll notice that there are a lot of boats grouped around the central cylinder of the blast. That close, they are tucked under the cloud.
Although it resembles a mushroom cloud, the sprawling cloud in the picture isn't caused by the same forces. It's actually the result of ionizing radiation moving through the atmosphere. The radiation ionizes the particles in the atmosphere, which then attract particles of water and cause large amounts of condensation - an actual cloud. The cloud and the radiation then rain down on the ships. (They are also exposed to direct radiation.)
Some of those ships were so undamaged that it was thought they could be saved. They were taken back to America - where they docked in Hunter's Point, San Francisco. Many of them, it turned out, could not be saved. They were irretrievably contaminated, among them the USS Independence. Although some reports say that the ships were dumped hundreds of miles from the coast, some locals say they could see the ships being sunk from shore.
Via The Devil's Teeth and Bulletin of the Atomic Scientists.