One of the longer-lived and more dangerous fission products dispersed globally by mid-20th century atmospheric nuclear weapons testing was 137Cs, which has a 30-year radioactive half-life. It has generally been assumed that outside of the vicinity of the test sites, 137Cs fallout washed off vegetation and was immobilized by soil, and thus of no ecological concern. Here we show that native plants thousands of kilometers from testing sites continue to cycle 137Cs because it mimics the essential nutrient potassium, and consequently, bees magnify this potentially lethal radionuclide in honey. There were no atmospheric weapons tests in the eastern United States, but, most honey here has detectable 137Cs at >0.03 Bq kg-1 (~1 million atoms per tablespoon), and in the southeastern U.S. it can be over 500 times higher. By measuring honey, we show regional patterns in the biogeochemical cycling of 137Cs for the first time and conclude that plants and animals receive disproportionally high exposure to ionizing radiation from 137Cs in soils with low potassium. In several cases, the presence of 137Cs more than doubled the ionizing radiation from gamma rays and x-rays in the honey, indicating that despite its radioactive half-life, the environmental danger to honeybees from this fission product can persist for more than six decades.