Stretching across northeastern Wyoming, the Powder River Basin is the largest coal-producing region in the United States, and it has made Wyoming the nation's leading coal state for decades. The scale of the operation is difficult to comprehend from the outside.

The basin's coal lies in seams so thick and close to the surface that it can be mined in enormous open pits rather than deep underground. Draglines the size of buildings strip away the earth, and the exposed coal is loaded onto trains more than a mile long that carry it to power plants across the country.

The coal here is prized for its low sulfur content, which burns cleaner than coal from many other regions and helped power plants meet clean-air rules. For years, that advantage made Powder River Basin coal the fuel of choice for a large share of the nation's electricity.

The industry has shaped the region utterly. Towns like Gillette grew on coal money, calling themselves the energy capital of the nation, and the mines have provided generations of well-paying jobs. Yet the basin now faces powerful headwinds as the country turns toward other energy sources. Its coal still lights millions of homes, but the question of how long that will last hangs over every mine and every family that depends on them.