Sonar mapping and seismic monitoring equipment will be used to map an underwater “forest” of prehistoric tree stumps recently discovered in Lake Michigan off Chicago. The U.S. Geological Survey, Northeastern Illinois University in Chicago, and the state geological surveys of Illinois and Indiana began mapping the area May 14. The primary mapping area will be about 2 square miles and is about 15 miles from the Chicago shoreline.

Scientists from the Illinois State Geological Survey and Northeastern Illinois University found that the stumps found so far are oak trees that grew about 8000 years ago; this finding could change previous assumptions about the geologic history of the lake.