Ajo

Town in Pima County, southwest Arizona, USA; population (2000) 3,700. It is 171 km/106 mi west of Tucson, and to the north of the Organ Pipe Cactus National Monument, in the Sonoran Desert. Copper, from deposits long known to the Tohono O'odham (Papago), was extracted commercially here from the 1910s, until dropping prices closed the mines in the 1980s. An open pit mine 2.4 km/1.5 mi across is to the south of the town. The New Cornelia Tailings Dam, composed of wastes piled so as to prevent the metal-heavy water washed from the mines from clogging streams, is variously described as the world's largest dam (in volume) and the world's largest mine waste dump.