The discovery of cosmic rays in 1912 was serendipitous. The “rays” turned out to be charged atomic nuclei but the name of the phenomenon, coined in the 30´s, has remained. The nature of the radiation was unravelled over some 40 years and led to discoveries from which the field of subatomic physics evolved. The sources of the most energetic cosmic rays, and the astrophysical processes which accelerate them, remain a mystery. Current research whick seeks to unveil the mysteries involves a huge detector in Argentina developed by an international collaboration on 17 nations.