Since the 1970s, mining companies have been using explosives and heavy machinery to demolish entire mountain peaks in West Virginia, Kentucky and other parts of the Appalachian region, in order to get at seams of coal that are difficult to reach with more conventional methods. Mountaintop removal mining, as the practice is called, has created jobs and income, but residents and environmental organizations say there’s been a heavy price.

In addition to destroying the natural beauty of the mountains, critics say that mountaintop removal destroyed forest ecosystems, contaminated groundwater with toxic chemicals, and caused other devastating damage.

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Now, a study by Duke University researchers, published in the journal Environmental Science and Technology, quantifies just how much that mountaintop removal has altered central Appalachia. By their calculations, there are areas 40 percent flatter than before the practice started, and the average slope in mined areas has been reduced by more than 10 degrees. The average elevation in mined areas actually has increased by about 10 feet, because of the filling of valleys with excavated material.

If you’re an urban dweller, the effect would be comparable to taking a wrecking ball to the local skyline and knocking a story off every building.

But those startling metrics don’t fully convey the impacts. The researchers say that by demolishing mountaintops and using the crushed rock to fill valleys, the mining technique has altered the region’s geology in other ways.

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“Mountaintop mining is penetrating much more deeply into the earth than other land use in the region like forestry, agriculture or urbanization,”  Emily Bernhardt, a professor of biology at Duke and co-author on the study, said in a press release. “The depth of these impacts is changing the way the geology, water and vegetation interact in fundamental ways that are likely to persist far longer than other forms of land use.”

The researchers found that by filling up the valleys with porous material changed the way that water moves through the area, and increases the chances that it will become contaminated by pollution from mining.

“You go from having shallow soil that is between half a meter and two meters deep, to something that is like a soil that is a hundred meters deep. The way the water moves through those two different landscapes is really different,” said Matthew Ross, an ecology Ph.D student and lead author on the study. “There are valley fills that are the size of an Olympic swimming pool and then there are valley fills that are 10,000 Olympic swimming pools, so there is a huge range in the capacity they have to hold water.”

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Some of the geological effects of mountaintop removal may last for thousands of years, Ross said.

At its peak in 2008, mountaintop removal, as the practice is called, produced more than 50 million short tons of coal a year, though since then, more stringent environmental regulation and a decrease in demand for coal has reduced production by more than half, according to the U.S. Energy Information Administration.