The pre-Columbian settlement at Cahokia was the largest city in North America north of Mexico, with as many as 20,000 people living there at its peak. Credit: Painting by Lloyd K. Townsend. Courtesy of the Cahokia Mounds State Historic Site, Illinois.
Last I checked, the Great Lakes and Gulf Coast were mostly in the U.S. Did some Aztecs wander in?
They were all immigrants who came over the Siberian land bridge 20,000 years ago.