Why did Machu Picchu fall into decline?

Related questions

1. Why did Machu Picchu take so long?
2. Does anyone live at Machu Picchu?
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5. Who built the Inca Trail?
6. Why is Machu Picchu a mystery?
7. Can you stay at Machu Picchu?
8. How long is the Inca Trail in miles?
9. What purpose did Machu Picchu serve?
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Tags

1. #spanish
2. #century
3. #america
4. #rulers
5. #appeared
6. #conquistadors
7. #along
8. #incan
9. #machu
10. #afflicting
11. #waged
12. #plagues
13. #capital
14. #their
15. #military
16. #picchu
17. #campaigns
18. #collapse
19. #survive
20. #south
21. #abandonment