The highest energy particles in Nature are cosmic rays. The most energetic yet detected has an energy of 3 x 1020 eV, a macroscopic energy greater than that of a tennis ball traveling at 100 mph. One of these particles hits on average a square kilometer only every century: so far, around 20 have been observed. We do not know how the particles are made, or whether they are photons, atomic nuclei or neutrinos, but a project now underway promises data that may let us answer such questions, and possibly lead us to new understandings of some important astrophysical problems - or even to some new physics.

That we know anything about such extraordinary particles is because of searches that were started many years ago. In 1938, the French scientist, Pierre Auger, discovered that showers of particles created in the atmosphere by an incoming cosmic ray were spread out over large distances at ground level.

To study these rare events in greater detail, an unusual observatory is taking shape in Mendoza Province, Argentina. Named the Pierre Auger Observatory, it is the focus of efforts by over 250 scientists from 15 countries. The Observatory consists of an array of 1600 water tanks overlooked by four massive photomultiplier cameras. It will cover 3000 km2, roughly the area of the state of Rhode Island. Shower particles are detected in the tanks using Cherenkov light and, on clear moonless nights, nitrogen fluorescence light seen by the cameras is used to trace the growth and decay of the cascades. Measuring cosmic ray air showers with both types of detectors makes the Auger Observatory a "hybrid detector." Hybrid measurements provide the best possible information about the primary particle composition. The Observatory will record about 50 events above 1020 eV every year. The major task of the Auger team is to measure the energy spectrum of cosmic rays above ~5 x 1019 eV with high precision.

The Auger Project calls for a matching site to be constructed also in the northern hemisphere. This will yield nearly uniform exposure to the entire celestial sphere. If the cosmic ray sources can be identified by clusters of arrival directions, then the Auger Observatory will study the sources' properties with equal sensitivity wherever they are in the sky.