The United States had chosen this string of islands halfway between Hawaii and Australia for its nuclear tests; specifically the atolls of Bikini and Enewetak. Each is a thin broken circle of coral reefs surrounding a lagoon, the remnants of ancient volcanoes. In the 1970s the United States was thinking about granting independence to the country, which it did eventually, and was considering what to do about the mess the testing left behind.

Bikini was so radioactive that there was little hope of allowing its displaced population ever to return home. But the military studied how to clean up Enewetak so that at least some land could become habitable again. The Defense Department concluded that there was so much soil contaminated with cesium-137 and strontium-90 that the safest approach was to leave it alone and let it decay naturally. Both have half-lives of about 30 years.

But also left behind by the blasts was plutonium-239, which has a half-life of 24,000 years. With enough plutonium-239 in the right form, a bomb could be made. That is why the United States participated in a $150 million operation, completed in 2012, to secure and clean up the plutonium at a Soviet-era nuclear test site in Kazakhstan.

At Enewetak, the United States decided in the late 1970s to dump as much plutonium-contaminated soil as it could gather into a 33-foot-deep crater on Runit that had been carved out in 1958 by a bomb roughly the size of the one detonated over Hiroshima.

In addition to the contaminated soil, crews filled 437 plastic bags with plutonium chunks they had picked up from the ground, left behind when one bomb misfired. These also went into the crater, which was then covered with an 18-inch-thick concrete cap. Most of the rest of the radioactive waste, with too little plutonium to trouble with, was bulldozed into the lagoon, over the objections of the Environmental Protection Agency and the displaced people of Enewetak. American officials also chose to leave radiation on the land at levels far higher than would be allowed after a similar cleanup in the United States.