I. Introduction
THE AIM of the Pierre Auger Observatory [1] is to discover and understand the origin of “ultrahigh energy cosmic rays” (UHECRs), above the GZK cutoff [2], [3]. A cutoff in the energy spectrum of primary cosmic rays around 6×10 eV is expected to be induced by the interaction of the particles with the 2.7°K primordial photons of the background radiation. Recent experimental data have shown that particles of 10 eV or higher exist and occur at a rate of about 1 per km2 per century. Detectors of very large acceptance are therefore needed to perform a statistically significant experiment. Moreover, in order to achieve nearly uniform exposure to the full celestial sphere, two matching sites, in the Northern and Southern hemispheres, should be instrumented. The Auger collaboration plans to instrument one site south of Mendoza, in Argentina, and the other in Millard County, UT (U.S.A). Work has begun in Argentina's Mendoza province. A sketch of the hybrid detector technique.