Jessie Barrett informed me of these unusual underwater structures in Wisconsin. The complete story can be found at
weirdus.com"The southern end of Rock Lake near Aztalan State Park in Wisconsin is the site of an unexplainable series of rock formations that has baffled everyone who has seen them for centuries.
When European settlers first took up residence in the area, Native Americans told them of rock teepees located on the lake's floor. In 1900, their existence was verified when local duck hunters caught site of the massive monoliths from their boat. Entirely submerged, the structures are 30 feet high and 100 feet long, and are conical in shape.
Explanations for the underwater stone pyramids vary. Some say that they are simply natural glacial formations, while others say that they are manmade, and were constructed by an ancient civilization before the area was flooded with water. Proponents of this theory point to the archeological evidence found nearby in Aztalan State Park, where the Mississippian Indians constructed pyramidal mounds over 1,000 years ago."