Super-high-energy galactic gamma rays have trillions of times more energy than visible light, and they disappear in the atmosphere before they hit the Earth’s surface. So if you want to detect these mysterious phenomena, a regular telescope isn’t much help. To learn about the highest-energy gamma rays, scientists build elaborate observatories, and one of the most advanced is the new High-Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory, high in the mountains about six hours from Mexico City. When it’s completed, it will be the only facility in the world that can detect the highest-energy gamma rays, with energies of up to...