Since cosmic rays were discovered in 1912, scientists have sought the origins of these mysterious particles. In September 2017, a flash of blue light in the ice deep beneath the South Pole set researchers on a path to resolving this century-old riddle. With the help of an icebound detector situated a mile beneath the South Pole, an international team of scientists has found the first evidence of a source of high-energy cosmic neutrinos, a ghostly subatomic particle that can travel in a straight line for billions of light years, passing unhindered through galaxies, stars and anything else nature throws in its path. The observation, made by the IceCube Neutrino Observatory at the Amundsen-Scott South Pole Station, helps resolve a more than century-old riddle about what sends subatomic particles such as neutrinos and high-energy cosmic rays speeding through the universe. Since they were first detected more than a hundred years ago, cosmic rays – highly energetic particles that continuously rain down on Earth from space – have posed an enduring mystery: What creates and propels the particles across vast distances? Where do they come from?