Mysterious Rock Wall of Rockwall
Rockwall, Texas
First uncovered in 1852, the rock wall that runs beneath this part of Texas was so baffling and well-known that both the county and town were named for it. The perfectly stacked-and-shaped rocks appear to be fashioned and mortared by human hands, but various scientists over the years have insisted that they're natural sandstone dikes. Others have compromised, suggesting that the "wall" is natural but that it may have been modified by a civilization so ancient that the entire structure was subsequently buried over centuries of time.
Part of the wall was excavated as a tourist attraction during the 1936 Texas Centennial. Another small section was dug up and reassembled behind the Rockwall County Historical Foundation building. It's the only part that's currently viewable to the general public, but apparently there's a lot more wall still underground.