The nation's rush to increase oil production is having a long-distance impact on the Great Lakes region.
Geologic formations have given parts of the region ample deposits of sand, including the hard, round version that is used in fracking. Seen from space a few months ago by the Landsat 8 satellite, the light brown mines dot a landscape of green fields and forests.
In the fracking process, a mix of water, sand and chemicals is pumped underground at high pressure, pushing into the cracks in rock layers. The mixture expands those cracks, allowing oil and natural gas to move more freely -- and be pumped by wells.
NASA's explainer notes that Wisconsin produces almost half of the sand used in fracking across the U.S. There were 73 active sand mines in the state last year; other facilities process sand to separate out the grains best suited for fracking.
About two-thirds of the nation's mined sand is used for fracking, NASA says. It's also used to make glass and in other industrial processes.
Most of Wisconsin's sand mines lie outside the Great Lakes basin, though some are close to Lake Michigan and Lake Superior.
Sand mining hasn't received as much attention as some other environmental issues in the Great Lake region. But if the oil boom continues, that's likely to change.