2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/2630A sudden steepening of the cosmic-ray energy spectrum (the knee) is observed at an energy of about 3 PeV (1 PeV = 10$^{15}$eV). The recent results on extensive air showers allow us to conclude that: a) the knee has an astrophysical origin; b) the 'sharpness' and the fine structure of the knee rule out 'Galactic Modulation' as the origin of the knee; c) most likely the knee is the result of the explosion of a single, recent, nearby supernova.4/ pages, 3 figures, submitted to Proceedings of Rencontres de Moriond, 'Very High Energy Phenomena in the Universe', Les Arcs 1800, January 20-27, 2001Astrophysicstext