Blue Hole: distant aerial

Blue Hole: map

Blue Hole: closer

Blue Hole: boats for scale

Blue Hole: scuba diving

Sink holes

Sink hole: Florida

Florida Winter Park sink hole

Early St. Louis, showing sink holes:

St. Louis sink hole: 1900

St. Louis: modern sink holes

Guatemala City “Sinkhole” – not actually a sinkhole

Blue Hole: former stalactites

Cenote:

Chichen Itza: map

Chichen Itza Cenote:

Chichen Itza Cenote:

Chichen Itza: aerial

Cenote Yax Mul, Mexico

Aricebo

Great Barrier Reef (Australia)

Great Barrier Reef (Australia)

Great barrier reef (Australia)

Zacaton (Mexico):

http://www.laputanlogic.com/articles/2004/02/17-0001.html

Zacaton (Mexico):

http://www.searchanddiscovery.com/documents/2008/08025franke/images/03.htm

Dean’s Blue Hole (Bahamas):

http://www.venturethere.com/blogs/detail/Freediving-Deans-Blue-Hole-Vid/186

Xiaozhai tiankeng

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

Xiaozhai tiankeng:

http://www.wondermondo.com/Countries/As/China/Chongqing/Xiaozhai.htm

Dashiwei tiankeng:

http://www.wondermondo.com/Countries/As/China/Guangxi/Dashiwei.htm

Dashiwei tiankeng:

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