The Pierre Auger Observatory is an international cosmic ray observatory in Argentina designed to detect ultra-high-energy cosmic rays: sub-atomic particles traveling nearly at the speed of light and each with energies beyond 10 eV. In Earth's atmosphere such particles interact with air nuclei and produce various other particles. These effect particles (called an "air shower") can be detected and measured. But since these high energy particles have an estimated arrival rate of just 1 per km per century, the Auger Observatory has created a detection area of 3,000 km (1,200 sq mi)—the size of Rhode Island, or Luxembourg—in order to record a large number of these events. It is located in the western Mendoza Province, Argentina, near the Andes.
Construction began in 2000, the observatory has been taking production-grade data since 2005 and was officially completed in 2008. The northern site was to be located in southeastern Colorado, United States and hosted by Lamar Community College. It also was to consist of water-Cherenkov detectors and fluorescence telescopes, covering the area of 10,370 km—3.3 times larger than Auger South.