The highest energy particles in Nature have been observed at earth, arriving from unknown sources. The energies are astounding -- subatomic particles with kinetic energies over 50 Joules -- so the accelerators at their sources must be among the most remarkable objects in the Universe. Yet background radiation renders the cosmos nearly opaque to their travel. Thus, the sources must be relatively nearby. We don't know what the particles are, where they came from, or how they achieved such high energy. A new international collaboration has begun construction of the Pierre Auger Observatory to solve the mystery. Two installations, in Argentina and the U.S., will each cover an area larger than Rhode Island with ground based detectors, in order to definitively measure the flux and map the arrival directions of these very rare events. The science, the evidence, the methods, and the opportunities will be discussed.

Further information: www.auger.org and links therein, especially www-lpnhep.in2p3.fr/auger/welcome.html