The Powder River Basin is a region in southeast Montana and northeast Wyoming about 120 miles east to west and 200 miles north to south known for its coal deposits. It is both a topographic drainage and geologic structural basin. The basin is so named because it is drained by the Powder River, although it is also drained in part by the Cheyenne River, Tongue River, Bighorn River, Little Missouri River, Platte River, and their tributaries. It 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. Major cities in the area include Gillette and Sheridan, Wyoming and Miles City, Montana. The area is very sparsely populated and is known for its rolling grasslands and semiarid climate. Because of the Powder River Basin, Wyoming is now the top coal producing state in the United States, a distinction that formerly alternated between West Virginia and Kentucky prior to the 1990s.