The Powder River Basin of northeast Wyoming and southeast Montana is region known for its coal deposits. Lying between the Black Hills and the Bighorn mountain range, the PRB is about 120 miles (193 km) east to west and 200 miles (322 km) north to south. The U.S. Bureau of Land Management says there are 13 active coal mines in the Wyoming portion of the Powder River Basin. These mines produced about 496 million short tons (450 million metric tons) as of September 2009. The PRB is the single largest source of coal mined in the United States and contains one of the largest deposits of coal in the world. Most of the active coal mining in the Powder River Basin actually takes place in drainages of the Cheyenne River. Because of the Powder River Basin, Wyoming has been the top coal-producing state in the United States since 1988.
The majority of the coal mined in the PRB is part of the Fort Union Formation. Because of its low sulfur and fly ash content, this coal is exported outside the region. In 2007, the Powder River Basin alone produced 436 million short tons (396 million tonnes) of coal, more than twice the production of second-place West Virginia, and more than the entire Appalachian region. Overall, the Powder River Basin accounts for about 37 percent of U.S. coal production.