The recent meteor explosion over Russia was a close call, but Iowa wasn’t so lucky 470 million years ago.

An ancient meteor gouged a 5.5 kilometer (3.4 mile) crater into was it now northeast Iowa. Aerial surveys recently confirmed the presence of the impact site, according to the U.S. Geological Society.

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Geologists from the Iowa Department of Natural Resources first found evidence of the crater in 2008. Rock from a water well boring operation revealed a circular layer of shale under the town of Decorah, Iowa. The tell-tale shale deposits were left by sediments from water that filled the crater after the impact.

A later study by the Smithsonian’s National Museum of Natural History found shocked quartz in the area, a stony smoking gun left by an extraterrestrial impact.

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The aerial survey that recently confirmed the craters’ existence used an electromagnetic system, which measured how well rocks conduct electricity, and a gravity gradiometer, which senses subtle changes in the Earth’s gravity. Computer models created using the data from these instruments showed a layer of pulverized rock beneath the shale and helped define the outline of the crater. The differences in the rock of the crater extend to a depth of several hundred meters.

IMAGE: The outline of the Decorah impact structure. (Adam Kiel, Northeast Iowa RC & D, USGS)