The tiny nation of Denmark has just three stations for monitoring atmospheric radiation. Each week, scientists change out air filters in the detectors and take the used ones to a technical university near Copenhagen. There, Sven Poul Nielsen and other researchers analyze the filters. They often snag small amounts of naturally occurring radioactivity, radon for example. Then about a month ago, Nielsen was startled to find something far stranger: a radioactive isotope known as ruthenium-106. Ruthenium-106 has a half-life of just one year, which means that it isn't naturally found on Earth. It is, however, created in the glowing cores of nuclear reactors — and usually only detected in the atmosphere when something goes terribly wrong. The ruthenium was detected far beyond Denmark. It showed up all across Europe, from Cyprus to Spain. The levels seen in dozens of nations were far too low to pose a health risk. Within weeks, the thin cloud had vanished. So far, no nation has announced a
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