Cosmic ray showers are cascades initiated by cosmic rays interacting in the atmosphere. Cascades had already been observed in the 1920s when a single track belonging to a charged particle was observed to split into two tracks. The observations of showers led to the development of the electromagnetic cascade theory in the 1930s with the participation of many of the famous physicists of the time. Towards the end of that decade Pierre Auger and collaborators discovered the extensive air showers [236]. These are huge cascades initiated by particles of energy 106 GeV as estimated in that reference.