The High Altitude Water Cherenkov Experiment or High Altitude Water Cherenkov Observatory (also known as HAWC) is a gamma-ray and cosmic ray observatory located on the flanks of the Sierra Negra volcano in the Mexican state of Puebla at an altitude of 4100 meters, at . HAWC is the successor to the Milagro gamma-ray observatory in New Mexico, which was also a gamma-ray observatory based around the principle of detecting gamma-rays indirectly using the water Cherenkov method.