Money that grows on crops: In the fragile Amazon River basin, for example, there are hundreds of artisanal mines where workers pour mercury, cyanide, and other chemicals onto gold-rich areas to extract the metal. Once the mine is exhausted, they abandon it and move on, leaving behind a toxic soup of contaminants.

"Basically a plant will take up anything that's in the soil," he says. Corn and canola have a natural ability to take up huge amounts of metal.

Of course, the crops aren't eaten because they're full of toxic metals. Instead, Anderson harvests them for their minerals as they begin to die. He estimates he can recover 14 ounces an acre and about half as much mercury through this process. Then the gold is used to pay for the cleanup and to educate locals about sustainable agriculture.

During the metal-harvesting, his team trains local people in farming techniques, so once the land is clean, they can reclaim it and use it for subsistence farming.