In Eritrea, Rossi discovered another phenomenon that would become a principal theme of his postwar cosmic ray research: extensive cosmic ray air showers. The discovery occurred during tests to determine the rate of accidental coincidences between the Geiger counters of his detector. To assure that no single particle could trigger the counters he spread them out in a horizontal plane. In this configuration, the frequency of coincidences was greater than that calculated on the basis of the individual rates and the resolving time of the coincidence circuit. Rossi concluded that: