The present white paper is submitted as part of the "Snowmass" process to help inform the long-term plans of the United States Department of Energy and the National Science Foundation for high-energy physics. It summarizes the science questions driving the Ultra-High-Energy Cosmic-Ray (UHECR) community and provides recommendations on the strategy to answer them in the next two decades.
Comment: Prepared as a solicited white paper for the 2021 Snowmass process. To be published in the Journal of High Energy Astrophysics. v2: fixed typos in author list. v3: included all community feedback received by July 1st 2022 and added the list of endorsers. v4 is the post-reviewer preprint accepted to Astroparticle Physics vol. 149