I will briefly review the observations of UHECRs, emphasizing which aspects are robust and which remain uncertain. Among the robust aspects, a key feature of the data strongly suggests that UHECRs are produced in BNS mergers.  Taking advantage of the well-defined initial conditions, I will identify the likely acceleration mechanism and explain how the spectral cutoff is determined.  The resulting no-free-parameter prediction of the cutoff energy for different nuclei agrees well with observations.  Unique predictions which can be confirmed within a decade or so, are that neutrinos above a PeV are preceded by a co-directional gravitational wave arriving about a day earlier, and that the highest energy cosmic rays originated from r-process nuclei and are heavier than iron.