The Pierre Auger Observatory is sited near the city of Malargüe, Argentina, at latitude 35.2 S. It is a hybrid instrument formed by an array of particle detectors and by fluorescence telescopes, used to measure the properties of extensive air showers produced by the highest-energy cosmic rays. The surface array comprises 1600 water-Cherenkov detectors deployed on a 1500 m triangular grid, covering about 3000 km2. The array is overlooked from four stations, each containing six telescopes used to detect the emitted fluorescence light.