A newly examined ice core shows what may be the chemical traces of supernovae that exploded a thousand years ago. Yuko Motizuki of the RIKEN research institute in Wako, Japan, and colleagues analysed the nitrate content of an ice core drilled at Dome Fuji station in Antarctica. Nitrate is produced in the atmosphere by nitrogen oxides, which in turn should be created by the gamma radiation from a supernova. Motizuki's group found high nitrate concentrations in three thin layers about 50 metres deep. Because snow gradually builds up into layers of ice, depth indicates age. After calibrating this icy calendar...