Puma Punku in Tiahuanaco - I do not think ''aliens'' were responsible for building this site, but I think ancient man was far more advanced than we as yet understand. The technology needed here was way beyond any later civilisations.
Puma Punku is a complex of ruins, part of a large temple group that is part of the Tiwanaku site near Tiwanaku, Bolivia ca 4000 m above sea level. Puma Punku is now understood to be at least 12,000 - 17,000 years old, essentially, right out of the Stone Age.
Walls around the Kalasasaya Temple. Tiwanaku (Spanish: Tiahuanaco ) a Pre-Columbian archaeological site in western Bolivia was the capital of an empire that extended into present-day Peru and Chile, flourishing from AD 300 to 1000.