Cosmic rays with energies above 1016.5 eV are studied through a hybrid-detection technique at the Pierre Auger Observatory, the world's largest detector arrangement for extensive air showers. Equipped with more than 1660 water-Cherenkov stations, the surface detector array (SD) spans an area of 3000 km2, while the combination of 27 fluorescence telescopes (FD) overlooks the atmosphere above the SD measuring the calorimetric energy of incoming particles. Currently, the observatory is undergoing a major upgrade of its capabilities, denominated AugerPrime. New detectors, such as radio antennas, surface scintillator detectors, and buried scintillator detectors are being deployed as part of this upgrade.
In this seminar, we present the hybrid detector of the Pierre Auger Observatory, along with the motivations and features of the upgrade. In particular, we describe the underground muon detector designed to measure the muon content of extensive air showers for cosmic ray primaries in the EeV region.