Did the mighty Aztec Empire actually begin in the heart of the American Midwest? Forensic geologist Scott Walter dives into the murky depths of Rock Lake, Wisconsin, to investigate centur-old legends of underwater pyramids and a mythical lake monster known as “Rocky.” By examining rare, suppressed maps from the 1800s and exploring the heavily fortified ruins of Aztalan State Park, Walter uncovers a startling connection between the Mississippian mound-builders and the civilization that would eventually rule Mexico.