  Ultrahigh-energy cosmic rays (UHECRs) arrive at Earth from the most energetic
astrophysical accelerators in the universe. They collide with atoms in the
upper atmosphere with energies about ten times higher than any man-made
accelerator, and produce gigantic cascades of secondary particles, called
extensive air showers (EAS).
  Extensive air showers can be detected spreading particle detectors over a
large area to record the interactions of secondary particles. The Pierre Auger
Observatory has been designed to investigate the origin and nature of UHECRs
using the combination of information from a surface array, measuring the
lateral distributions of secondary particles at the ground, and fluorescence
telescopes, observing the longitudinal profile of the electromagnetic component
of EAS, providing an enhanced reconstruction capability.
  In this contribution, the status and prospects of understanding the physics
of UHECRs will be reviewed, focusing on the progress made thanks to the
measurements of the Pierre Auger Observatory. Physics results from the
ultrahigh-energy cosmic ray data collected with the Pierre Auger Observatory
opened new perspectives and motivated an upgrade of the Observatory,
AugerPrime, whose main characteristics are also presented.
