Five thousand miles west of Los Angeles and 500 miles north of the equator, on a far-flung spit of white coral sand in the central Pacific, a massive, aging and weathered concrete dome bobs up and down with the tide.

Here in the Marshall Islands, Runit Dome holds more than 3.1 million cubic feet — or 35 Olympic-sized swimming pools — of U.S.-produced radioactive soil and debris, including lethal amounts of plutonium. Nowhere else has the United States saddled another country with so much of its nuclear waste, a product of its Cold War atomic testing program... [+]