The Mayans
The Mayan civilization is one of the most powerful civilizations in the world between the 8th and 9th centuries. Their civilization is recognized for their calendar systems, hieroglyphic writing, and achievements in agriculture and architecture.
However, around 900 A.D., almost all the major cities of the Mayan civilization were abandoned. Nobody knows why. Some scientists and historians credit it to a mixture of factors like warfare, overpopulation, shifting trade routes, and extended drought. However, the true, proven cause is still unknown to this day.