WRIGHT, Wyo. -- The first thing you notice about the active surface mining operations in Wyoming's Powder River Basin is the size of it all.

The vast rugged prairie, uninterrupted by trees, where the mines are dug.

The machines, so massive they have to be built on site,

Everything is over-sized in the coal mines of northeastern Wyoming. This truck is capable of holding up to 340 tons of coal, enough to load three rail cars. It costs about $3.5 million.

that claw away hundreds of thousands of tons of the prairie to expose its ancient black underbelly.

The elaborate dump trucks, five stories tall, that haul the coal to processing and loading facilities.

The 100-car trains that slide across the land like fat snakes.

The millions of years it took nature to form this solid fuel and the ease with which it's being hauled away.

Coal can be found, at least in small amounts, in nearly every state and is mined in 27 of them (Minnesota isn't one of them). But the most productive coal mines are in the states of Wyoming, West Virginia and Kentucky, in that order. No other state comes remotely close to Wyoming's production.

And it is Wyoming that today is the blockbuster star of the solid fuels industry. Not only does the state possess some of the nation's most vast and accessible coal reserves, but most of the coal mined here has a low sulfur content.

That means that when it is burned in power plants, it emits lower levels of sulfur dioxide, one of the primary culprits responsible for acid rain, than coal from most other regions of the country. And sulfur emissions are an important issue for power companies as they attempt to comply with federal pollution-control standards that are being phased in during the next two decades.

Although most of the coal in the basin is low in sulfur, it varies widely in heat value, depending on where it's mined. Coal at the Black Thunder mine in the southern part of the basin, for example, is high in heat value and sells for a higher price than coal mined further to the north.