They detect a cascade of secondary particles created from the collision of cosmic rays with the upper atmosphere. There are a lot more of them. https://en.wiki

They detect a cascade of secondary particles created from the collision of cosmic rays with the upper atmosphere. There are a lot more of them. https://en.wikipedia.org/wiki/Air_shower_(physics) https://en.wikipedia.org/wiki/Air_shower_(physics) https://en.wikipedia.org/wiki/Cosmic-ray_observatory https://en.wikipedia.org/wiki/Cosmic-ray_observatory