The Pierre Auger Observatory is an international cosmic ray observatory in Argentina designed to detect ultra-high-energy cosmic rays: sub-atomic particles traveling at the speed of light and each with energies beyond 10¹⁸ 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 km² per century, the Auger Observatory has created a detection area of 3,000 km² (1,200 sq mi) — the size of Rhode Island, or Luxembourg — in order to record a large number of these events.