Cosmic ray particles interact mostly with nitrogen molecules in our atmosphere to produce secondary particles. As these continue on their pathway toward earth, they interact again, each one creating even more lower energy particles, to produce a cascade.

Once the cosmic ray cascade reaches an elevation of around 3,500 to 4,000 feet, the original particle has created a cascade of millions or perhaps even billions of secondary particles that can be detected. This cascade can be spread over an area of 10-20 square miles for the highest energy particles. By putting detectors over a large area, scientists hope to intercept as many particles as possible from this cascade, thus enabling them to receonstruct the original particle trajectory, mass and energy. Once the cascade goes below about 3500 ft, the number of particle begins to dissapate and it is harder for scientists to use the secondary particle information.