28. Noone Knows The Cause of Their Decline
After flourishing for over 3000 years, the Mayan civilization saw a rapid and mysterious decline in the 8th and 9th century AD. This period saw most of the main Mayan cities getting abandoned and their populations steadily falling.
There are over 80 theories about the possible reasons for their downfall, though historians cannot agree on a universally accepted reason. Some of the theories mention foreign invasion, the collapse of trade routes, or disease, and famine as possible reasons for the collapse.