2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/2317These lectures discuss the mysteries involving the production and extragalactic propagation of ultrahigh energy cosmic rays and suggested possible solutions.Lectures given at the D. Chalonge Euroschool, Erice, Italy, November 2000, 25 pages, 7 ps figs., expanded revision with color fig. 7AstrophysicsHigh Energy Physics - Phenomenologytext