Earlier this month, two sisters fell into a mine shaft in Arizona while riding an all-terrain vehicle. It was a terrible tragedy, but, unfortunately, not an unexpected one: an estimated 500,000 abandoned mines litter the U.S., mostly in the West. Even though the oldest mines were closed almost a century ago, many are still leaching heavy metals into groundwater — according to one expert, 40 percent of Colorado’s waterways are affected by mine runoff. Private landowners often aren’t aware of mines on their property, and sprawling communities are only encroaching nearer to dangerous areas. The U.S. EPA and U.S. Forest Service are responsible for cleanup, but, oddly enough, there never seems to be quite enough funding.