What are EAS? Extensive Air Showers or EAS are caused by very energetic particles interacting with air molecules high in the atmosphere. A gamma ray photon or proton will hit an air molecule producing several secondary particles, these secondary particles then interact to create even more tertiary particles, etc. A cascade develops in the atmosphere as you can see by taking the tour. These cascades can be detected by ground based particle detectors like Milagro or by looking at the light produced in the atmosphere as Whipple and other Air Cherenkov Detectors do.