The Chicago Air Shower Array (CASA) was a significant ultra high high-energy astrophysics experiment operating in the 1990s. It consisted of a very large array of scintillation detectors located at Dugway Proving Grounds in Utah, USA, approximately 80 kilometers southwest of Salt Lake City. The full CASA detector, consisting of 1089 detectors began operating in 1992 in conjunction with a second instrument, the Michigan Muon Array (MIA), under the name CASA-MIA. MIA was made of 2500 square meters of buried muon detectors. At the time of its operation, CASA-MIA was the most sensitive experiment built to date in the study of gamma ray and cosmic ray interactions at energies above 100 TeV (1014 electronvolts). Research topics on data from this experiment covered a wide variety of physics issues, including the search for gamma rays from Galactic sources (especially the Crab Nebula and the X-ray binaries Cygnus X-3 and Hercules X-1) and extragalactic sources (active Galactic nuclei and gamma-ray bursts), the study of diffuse gamma-ray emission (an isotropic component or from the Galactic plane), and measurements of the cosmic ray composition in the region from 100 to 100,000 TeV. For the topic of composition, CASA-MIA worked in conjunction with several other experiments at the same site: the Broad Laterial Non-imaging Cherenkov Array (BLANCA), the Dual Imaging Cherenkov Experiment (DICE) and the Fly's Eye HiRes prototype experiment. CASA-MIA operated continuously between 1992 and 1999. In summer 1999, it was decommissioned. Specifications and design CASA was built to study the possibility of astrophysical sources of ultra high energy (UHE, E > 100 TeV) gamma-ray emission (see Science below). Gamma rays at these energies interact in the Earth's atmosphere to create an extensive air shower that propagates to the Earth's surface. At the surface, the shower consists predominantly of electrons/positrons, low-energy gamma rays, muons, and some hadrons, with a typical footprint on the ground of 50–100 m. (There is also a component of Cherenkov radiation reaching the ground that can be recorded by imaging atmospheric Cherenkov telescopes). An air shower array is a distributed set of particle detectors (scintillation detector, water Cherenkov detectors, etc.) spread out on the ground to record the passage of the shower particles. The primary particle direction is estimated from the relative arrival time of the shower hitting each detector; the primary particle energy is estimated from the number of particles recorded in each detector and from the lateral distribution