7. Khmer Empire
The Khmer Empire, centered in present-day Cambodia, is best known for the awe-inspiring Angkor Wat temple complex. Though the empire flourished for centuries, its mysterious decline by the 15th century remains unexplained.
Theories range from environmental factors, such as flooding or drought, to internal conflict or invasions. Despite its fall, the grandeur of Angkor continues to captivate visitors worldwide.