Cahokia Mounds (IMAGE) Washington University in St. Louis Caption The remains of the most sophisticated prehistoric native civilization north of Mexico are preserved at Cahokia Mounds State Historic Site in Illinois. Archaeologist Natalie Mueller's new study casts doubt on a popular theory about why the ancient city was abandoned. Credit Photo: Joe Angeles / Washington University Usage Restrictions For editorial purposes License Original content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.