The Pierre Auger Observatory is an international scientific project with the objective of studying the highest energy cosmic rays.
Cosmic rays of energy of the order of 10**20 eV have been observed. The origin and accelerating process of these particles is still unknown. Their rate is extremely low. One expects approximately one cosmic ray arriving on an area of one km2 per century.
In order to collect a significant statistics, the Pierre Auger Observatory covers an area of 3000 km2. The cosmic rays properties are measured by two independent detector systems. The Surface Detector is a giant array of 1600 water Cherenkov tanks, placed over the area with 1.5 km spacing.
The Fluorescence Detector is a telescope system which reconstructs the cosmic ray shower from the fluorescence light emitted by the atmospheric nitrogen excited by the particles of the shower.
Surface arrays measure the lateral distribution of particles in air showers when they strike the ground. Fluorescence detectors view the longitudinal development of showers as they move downward through the atmosphere. These two complementary techniques form a uniquely powerful instrument to study the nature of extreme energy cosmic rays.
The Southern Observatory is under costruction in the Pampa Amarilla near the small town of Malargue (Argentina). It is expected to measure the arrival direction, the energy and mass composition of primary cosmic rays collecting about 5000 events per year above 10**19 eV.
The 'Engineering Array', a first element of the Observatory, composed of 40 tanks and two fluorescence detector telescopes was operated for a few months between December 2001 and April 2002. During these few months of operation we collected about 100 hybrid events, seen simultaneously by the surface and by the fluorescence detectors.
The Engineering Array is only a small fraction in size (few per cent) of the full Observatory but it includes all elements of the project, surface and fluorescence detectors, calibration systems, communication array and data acquisition.
The operation of the Engineering Array allowed a general test on the field of all components to be made.
The success of the operation of the Engineering Array has given confidence on the validity of the choices made on the construction of the detectors. Several minor but relevant modifications have been done with respect to the original design.
The installation of the surface detectors and of the fluorescence telescopes is progressing at the expected rate.
Data taking and analysis is also taking place while the Observatory is expanding to reach the final design configuration.
The first scientific results were presented at the International Conference on Cosmic Rays (ICRC2005 in Pune, India).
Completion of the Observatory is foreseen for the end 2007.
The basic motivations for the AUGER Observatory can be listed as follows:
In the cosmic radiation particles exist with energies around 10**20 eV (50 joules).
There is no understanding how known astrophysical objects can accelerate particles to such enormous energies.
Because of interaction with the 2.7 K cosmic background radiation the sources must be close on a cosmological scale (distance less than about 50 Mpc or 150 million light years).
These particles are little affected by the magnetic field of our galaxy or the extra-galactic magnetic field. Their arrival direction should point to their source giving rise to a new form of astronomy.

The prospect for the discovery of new physics or astrophysics is likely.