  The Global Cosmic-ray Observatory (GCOS) is a proposed large-scale
observatory for studying ultra-high-energy cosmic particles, including
ultra-high-energy cosmic rays (UHECRs), photons, and neutrinos. Its primary
goal is to characterise the properties of the highest-energy particles in
Nature with unprecedented accuracy, and to identify their elusive sources. With
an aperture at least a ten-fold larger than existing observatories, this
next-generation facility should start operating after 2030, when present-day
detectors will gradually cease their activities. Here we briefly review the
scientific case motivating GCOS. We present the status of the project,
preliminary ideas for its design, and some estimates of its capabilities.

High Energy Astrophysical Phenomena;Cosmology and Nongalactic Astrophysics;Instrumentation and Methods for Astrophysics;High Energy Physics - Experiment;Instrumentation and Detectors