After burning coal to light up Cincinnati for six decades, the Walter C. Beckjord Generating Station will go dark soon—a fate that will be shared by dozens of aging coal-fired power plants across the U.S. in coming years.

Their owners cite a raft of new air-pollution regulations from the Environmental Protection Agency, including a rule released Wednesday that limits mercury and other emissions, for the shut-downs.

But energy experts say there is an even bigger reason coal plants are losing out: cheap and abundant natural gas, which is booming thanks to a surge in production from shale-rock formations in the U.S.

"Inexpensive natural gas is the biggest threat to coal," says Jone-Lin Wang, head of global power research for IHS CERA, a research company. "Nothing else even comes close."

For decades, coal produced more electricity than all other fuels combined, and as recently as 2003 accounted for almost 51% of net electricity generation, according to the U.S. Energy Information Administration.