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 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. 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.

When construction began, a full-scale prototype was set up first: the Engineering Array. This array consisted of the first 40 ground detectors and a single fluorescence detector. All were fully equipped. The engineering array operated for 6 months in 2001 as a prototype; it was later integrated into the main setup. It was used to make more detailed design choices (like which type of photomultiplier tube (PMT) to use, and tank water quality requirements) and to calibrate.

In 2003, it became the largest ultra-high energy cosmic ray detector in the world. It is located on the vast plain of Pampa Amarilla, near the town of Malargüe in Mendoza Province, Argentina. The basic set-up consists of 1600 water Cherenkov Detectors or 'tanks', (similar to the Haverah Park experiment) distributed over 3,000 square kilometres (1,200 sq mi), along with 24 atmospheric Fluorescence Detector telescopes (FD; similar to the High Resolution Fly's Eye) overseeing the surface array.

The observatory was named after the French physicist Pierre Victor Auger. The project was proposed by Jim Cronin and Alan Watson in 1992. Today, more than 500 physicists from nearly 100 institutions around the world are collaborating to maintain and upgrade the site in Argentina and collect and analyse the measured data.

According to wikipedia