At the north end of Louisiana State University's (LSU) campus sit two grassy mounds, rising in a gentle slope to a height of about 20 feet (6 meters). The mounds are just two of more than 800 similar human-made mounds in Louisiana, built by Indigenous Americans. Although researchers knew they were old, a new study has determined just how old these ancient structures are.

The grassy surface hides layers of ancient clay, dirt and ash. And researchers recently found that the oldest mound is 11,000 years old, making it the oldest human-made structure discovered in either North or South America.

https://www.livescience.com/oldest-human-made-structure-americas