Map of the North American Laurentian Great Lakes Basin showing regions of karst limestone formations and regions of above ground limestone formations in Alpena County, MI and submerged sinkholes (including the study sites – Misery Bay containing the El Cajon Bay Blue Hole, Middle Island Sinkhole and Isolated Sinkhole) in the Thunder Bay National Marine Sanctuary (TBNMS), Lake Huron.
Image courtesy of Thunder Bay Sinkholes 2008, NOAA, OceanExplorer.noaa.gov