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.