Answer: The Mayan civilization was not only vast but also developed beyond its time. This included the temples and pyramids that were difficultly engineered. The Incan civilization is credited with the development of roads and trade routes, making them a significant "nation" especially considering the age (of time). The Aztec civilization developed astonishingly accurate measurements of time and astronomy. These three civilizations are regarded as "Great" because of their extraordinary advancements (given the time period).
