Pierre Auger, who had positioned particle detectors high in the Alps, noticed that two detectors located many metres apart both signaled the arrival of particles at exactly the same time. A systematic investigation of the showers showed coincidences between counters separated horizontally by as far as 75 metres. While the counting rate dropped sharply in going from 10 centimetres to 10 metres, the rate decreased slowly at larger distances.

Auger had recorded "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 – 10 million times higher than any known before.