A neutrino hit on September 22, 2017, at 4:54 P.M. Eastern time. The nearly massless elementary particle barreled through the sensors of the IceCube Neutrino Observatory, an experiment buried in the Antarctic ice. This neutrino was rare, carrying an energy of more than 100 tera electron volts, about 10 times the energy reachable by particles inside the most powerful accelerators on Earth. Thirty seconds later IceCube’s computers sent out an alert with the neutrino’s energy, the time and date, and roughly where it came from in the sky.