By Elizabeth Hernandez and Eric Chaney

Up close, the biggest emitter of greenhouse gases in the U.S. isn't as big as you'd expect it to be. From most angles, you can't even see it until you're right on top of it.

But hit the right gap in the rolling hills of north-central Alabama, and the James H. Miller Jr. Electric Generating Plant looms large even from miles away. Nestled on about 800 acres on the Locust Fork of the Black Warrior River, the plant is one of Alabama Power's coal-burning workhorses, putting out enough electricity to power about a million homes. It virtually never stops running—and never stops producing carbon dioxide and other greenhouse gases.