When a super-high-energy particle hits the top of the atmosphere, it (thankfully) doesn’t make it all the way down to the Earth’s surface without banging into a lot of air and gas molecules. When a 1020 eV particle first hits a molecule in the atmosphere, it breaks up into two particles each with half that energy. Those two particles then hit other molecules, creating four particles with a quarter the energy, and so on. Eventually, you get trillions of particles with 109 eV of energy washing over the surface of the Earth in a flash. This shower of particles is typically about a kilometer or
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