Greisen–Zatsepin–Kuzmin effect: top-down and bottom-up view

In 1966, Greisen and independently Zatsepin and Kuzmin published evidence for the existence of a relic (GZK) cutoff in the proton energy spectrum in ultra-high energy cosmic rays (UHECRs) above $5 \times 10^{19}$ eV. Half a century of experimental ground-based UHECR research has resulted in a large amount of data on energy spectra, anisotropy, and mass composition. The first space experiment to measure UHECRs was launched in 2016. In this paper, we discuss the results and prospects of experimental UHECR research in light of the proposed theoretical model of the GZK cutoff.