ASARCO Mission Mine and Mineral Discovery Center

One of the largest open-pit copper mining operations in the entire country, ASARCO is one of the only Arizona giants to provide a tour. Highly recommended if you're in the Tucson area, the tour welcomes both those in favor of and opposed to open-pit copper mining and its environmental consequences.

The Mineral Discovery Center screens a short film called "Mining for Music" and the science exhibits and displays of historic mining equipment from what was once Arizona's biggest industry are impressive in scope and scale. Some of the exhibits demonstrate how copper deposits are formed naturally in the earth, how miners extract copper minerals from the rock to produce nearly pure metal, and more. The Mission Mine has an average grade of only about 0.67 percent copper, which means that every ton - or 2,000 pounds - of ore that comes out of the mine produces only 13 pounds of copper.

In addition, for every ton of ore, about three tons of waste rock must be removed. To keep the pit economically feasible, engineers use sophisticated equipment that tracks progress. When the price of copper is low, miners work on excavating areas with higher-grade ore. The Mission Mine is about two miles across and a quarter-mile deep. About six times the amount of earth that was moved to form the Panama Canal has been mined at this site.