  Ultra-high-energy cosmic rays (UHECRs) are the highest energy messengers in
the universe, with energies up to $10^{20}$ eV. Studies of astrophysical
particles (nuclei, electrons, neutrinos and photons) at their highest observed
energies have implications for fundamental physics as well as astrophysics. The
primary particles interact in the atmosphere (or in the Earth) and generate
extensive air showers. Analysis of those showers enables one not only to
estimate the energy, direction and most probable mass of the primary cosmic
particles, but also to obtain information about the properties of their
hadronic interactions at energies more than one order of magnitude above that
accessible with the current highest energy human-made accelerator. The Pierre
Auger Observatory, located in the province of Mendoza, Argentina, is the
largest cosmic ray experiment ever built. The observatory was designed as a
hybrid detector covering an area of 3000 km$^2$ and has been taking data for
almost twenty years. In this paper, a selection of the latest results is
presented: the cosmic ray energy spectrum, studies of hadronic physics,
searches for a directional anisotropy and studies of mass composition
(including the photon and neutrino searches). Finally, the current upgrade
("AugerPrime") of the observatory, which is mostly aimed at improving the
sensitivity to the particle type and mass of ultra-high energy cosmic rays, is
described.
