Scientists long tried to figure out why some spots on Earth had more radioactive air than others. It wasn’t until 1912, when Victor Hess took an electrometer skyward in a balloon, that it became clear the extra radiation was coming, not from inside the Earth, but from above it. Way above it. But where exactly did these “cosmic rays,” as physicist Robert Millikan dubbed them, come from? (The rays are, more precisely, high-energy charged particles.) An international network of astronomers, including researchers from McGill, just might have a lead on this mystery.