What is an extensive air shower caused by a cosmic ray?

An extensive air shower (EAS) is a cascade of secondary particles generated when a very high-energy cosmic ray primary particle strikes the atmosphere.12

When a primary particle with energy exceeding approximately ${10}^{18}$ electron volts enters the atmosphere, it triggers a chain reaction of collisions that produces a vast number of secondary particles. An EAS can consist of billions of these secondaries, including photons, electrons, muons, and neutrons.1

These showers spread out as they descend, eventually arriving at ground level over an area spanning many square kilometers. Because these high-energy primary particles are extremely rare, scientists detect them by deploying large arrays of particle detectors over wide areas to capture the footprint of these extensive showers.1