Pierre Auger, who had positioned particle detectors high in the Alps, noticed that two detectors located many meters apart both signaled the arrival of particles at exactly the same time. Auger had discovered "extensive air showers," showers of secondary subatomic particles caused by the collision of primary high-energy particles with air molecules. On the basis of his measurements, Auger concluded that he had observed showers with energies of 1015 eV -- ten million times higher than any known before.