The High Resolution Fly's Eye or HiRes detector was an ultra-high-energy cosmic ray observatory that operated in the West Desert of Utah from 1981 until April 2006. HiRes used the "atmospheric fluorescence" technique that was pioneered by the Utah group first in tests at the Volcano Ranch experiment and then with the original Fly's Eye experiment. The experiment first ran as the HiRes prototype in a tower configuration operating in conjunction with the CASA and MIA. The prototype was later reconfigured to view 360 degrees in azimuth. HiRes-II followed later and was located on a hilltop about 13km away. HiRes-I and HiRes-II operated in stereo. In 1991 it discovered the Oh-My-God Particle.