Formed deep underground over thousands of years of heat and pressure, coal is a carbon-rich black rock that releases energy when burned. In the United States, roughly 30 percent of all electricity comes from coal: the rest comes from natural gas, nuclear, and renewables like wind and solar.

Most US coal comes from either above-ground mines in Wyoming and Montana, or underground mines in Appalachia. Mining jobs—once an important component of the US economy—have fallen dramatically since peaking in the 1950s, thanks in large part to advances in machine automation.

More recently, coal production and coal power have slowed, as other cleaner sources of electricity become more affordable. While difficult for the communities that depend on coal mines and plants, the transition away from coal is essential for avoiding some of coal’s worst impacts.