7. The Khmer Empire (Angkor)
The Khmer Empire created Angkor Wat, one of the largest religious monuments in the world. From the 9th to 15th century, it ruled much of Southeast Asia before mysteriously declining.
Why They’re Mysterious:
- Built giant temples surrounded by moats.
- Mastered water management systems.
- Possible reasons for collapse: droughts, wars, or political shifts.