Cosmic rays are so energetic that they pass through just about any obstacle, including Earth itself. Occasionally, though, they will collide with matter in Earth’s atmosphere; the interaction causes a cascade of electromagnetic radiation to spew forth, which is called a Cherenkov shower. To study these powerful cosmic rays, astronomers have built “atmospheric Cherenkov detectors.” By analyzing the showers, scientists can deduce important properties about the cosmic rays that created them.