the coal seams near the surface. Coal Powder River Basin (PRB) coal is classified as "sub-bituminous" and contains an average of approximately 8,500 btu/lb, with low sulfur. Contrast this with eastern, Appalachian bituminous coal containing an average of 12,500 btu/lb and high sulfur. PRB coal was essentially worthless until air pollution emissions from power plants (primarily sulfur dioxide, or "SO2") became a concern. A coal-fired plant designed to burn Appalachian coal must be modified to remove SO2 at a cost estimated in 1999 to be around $322 per ton of SO2. If it switched to burning PRB coal, the cost dropped to $113 per ton of SO2 removed. Removal is accomplished by installing scrubbers. The Powder River Basin is the largest coal mining region in the US, but most of the coal is buried too deeply to be economically accessible. The Powder River Basin coal beds are shaped like elongated bowls and as mines expand from east to west in the Powder River Basin, they will be going "down the sides of the bowl". This means that the overburden (rock lying over the coal) will increase as will the stripping ratio (the ratio of rock that needs to be moved to produce a ton of coal). The United States Geological Survey (USGS) has conducted a series of studies on the economic accessibility of coal in the major coal-producing regions of the country. The studies typically found that only a small fraction of the coal would be economically accessible at the price then of $10.47/ton. In August 2008, the USGS issued an updated assessment of coal in the Powder River Basin. After considering stripping ratios and production costs, the USGS concluded that at that time, only 6% of the original resource, or 10.1 billion short tons of coal, was economically recoverable. At a price of $60/ton, however, roughly half (48%) of the coal would become economic to produce. Increasing the price paid for coal can increase the amount of economically recoverable coal, but increasing the price of coal also increases its production cost. Because coal is a solid, it cannot be produced from many scattered wells like oil and gas can be. Rather, coal has to be produced from mines that expand slowly by moving massive quantities of overburden. Coal mining Fifteen mines operate in the Powder River Basin, with most of the active mining taking place in drainages of the