Shows that a GUT magnetic monopole’s core acts as a catalyst for baryon-number-violating processes, so a proton passing near a monopole can decay at a rate far above the ordinary GUT-suppressed rate — the founding result of monopole-catalyzed proton decay (the “Callan-Rubakov effect”). relevance_note: establishes the mechanism by which a magnetic monopole (if collider-produced) could in principle catalyze destruction of ordinary matter — the theoretical basis of the monopole-doomsday route.