1. Blue Hole: distant aerial

2. Blue Hole: map

3. Blue Hole: closer

4. Blue Hole: boats for scale

5. Blue Hole: scuba diving

6. Sink holes

7. Sink hole: Florida

8. Florida Winter Park sink hole

9. Early St. Louis, showing sink holes:

10. St. Louis sink hole: 1900

11. St. Louis: modern sink holes

12. Guatemala City “Sinkhole” – not actually a sinkhole

13. Blue Hole: former stalactites

14. Cenote:

15. Chichen Itza: map

16. Chichen Itza Cenote:

18. Chichen Itza: aerial

19. Cenote Yax Mul, Mexico

20. Aricebo

21. Great Barrier Reef (Australia)

23. Great barrier reef (Australia)

24. Zacaton (Mexico): http://www.laputanlogic.com/articles/2004/02/17-0001.html

25. Zacaton (Mexico): http://www.searchanddiscovery.com/documents/2008/08025fr anke/images/03.htm

26. Dean’s Blue Hole (Bahamas): http://www.venturethere.com/blogs/detail/Freediving-Deans- Blue-Hole-Vid/186

27. Xiaozhai tiankeng http://www.speleogenesis.info/archive/publication.php?Type= publication&PubID=3289

28. Xiaozhai tiankeng: http://www.wondermondo.com/Countries/As/China/Chongqin g/Xiaozhai.htm

29. Dashiwei tiankeng: http://www.wondermondo.com/Countries/As/China/Guangxi/D ashiwei.htm

30. Dashiwei tiankeng: http://punjapit.wordpress.com/2011/02/08/chinathe-dashiwei- tiankeng/