RED WING, Minn. (AP) — Under the forested bluffs of southeastern Minnesota lies an increasingly sought-after resource. It’s called “frac sand.” And it’s prized by the energy industry, which uses it to extract gas and oil from underground rock in a process called hydraulic fracturing, or fracking.

Minnesota Public Radio reports an increasing number of companies are eying Minnesota for the sand, prized for its perfectly round, hard and chemically inert grains. One energy company recently purchased land near Red Wing for sand mining, sparking opposition from residents and environmentalists.

There are already a handful of frac sand mining operations in the region. And Tony Runkel, the state’s chief geologist, says the Mississippi River Valley in parts of Minnesota, Wisconsin and Illinois has the potential to turn into a hot spot for them.