Pierre Auger Observatory
The Pierre Auger Observatory is an NSF-supported enormous array of detectors in Argentina at the foothills of the Andes. Its aim is the study of the rarest and most energetic particles in the Universe since the big bang: cosmic rays (most likely protons or bare atomic nuclei) at extremely high energies, whose origins are still a deep mystery despite decades of intense investigations worldwide. Physicists from 15 countries have been deploying detectors over an area of some 3000 km^2 in western Mendoza province to detect these rare air showers. With these detectors the international Collaboration has measured the energy distribution of these particles, has studied their arrival directions from the cosmos, has studied the particle physics details of their atmospheric interactions, has searched for neutrino, gamma ray and magnetic monopole candidates at the highest energies, and is planning upgraded detectors to assess the types of atomic nuclei making up the cosmic ray flux at these energies. These are the clues needed to finally resolve the puzzle of their origins.