The 'mystery' of who built the earthen mounds in the Midwest was nothing but white dude propaganda
I've lived my whole life as a pale, red headed fella. So, I say this, with authority: white people are dicks.
According to Smithsonian.com, white pioneers and archeologists in the 18th and 19th centuries pumped out a bullshit story about Cahokia, once the largest Native American city north of Mexico, as having been built by the Welsh, Vikings, Hindus - anyone but the indigenous population:
The city of Cahokia is one of many large earthen mound complexes that dot the landscapes of the Ohio and Mississippi River Valleys and across the Southeast. Despite the preponderance of archaeological evidence that these mound complexes were the work of sophisticated Native American civilizations, this rich history was obscured by the Myth of the Mound Builders, a narrative that arose ostensibly to explain the existence of the mounds. Examining both the history of Cahokia and the historic myths that were created to explain it reveals the troubling role that early archaeologists played in diminishing, or even eradicating, the achievements of pre-Columbian civilizations on the North American continent, just as the U.S. government was expanding westward by taking control of Native American lands.
So yeah: it's hard to claim that you're displacing or irradiating a gaggle of savages when they prove themselves to be part of a society with a culture and history that's just as complex as your own.
Cahokia's collection of earthen mound structures aren't the only ones said to have been created by a mysterious group of builders. Similar sites can be found all over Ohio, the Mississippi Valley and well into the Southeast.
Image: Skubasteve834 - EN.Wikipedia, CC BY-SA 3.0, Link