The Powder River Basin sports a colorful history. Bones of bison slaughtered by people, found south of Sundance, Wyo., date back 6,000 years, and northeast Wyoming remained a favorite hunting ground for American Indians into the late 19th century. At that time the Powder River Basin was the scene of violent conflicts between the Indians and U.S. military men. Abundant grass made this region a favored spot for cattle and sheep ranchers. Under the grass is coal—so much of it that about 40 percent of the coal mined in the U.S., comes from the Powder River Basin.