Coal ash, which contains many of the world's worst carcinogens, is what's left over when coal is burnt for electricity. An estimated 113 million tons of coal ash are produced annually in the US, and stored in almost every state — some of it literally in people's backyards. With very little government oversight and few safeguards in place, toxic chemicals have been known to leak from these storage sites and into nearby communities, contaminating drinking water and making residents sick.