What great mysteries lurk unseen beneath your very feet? Introducing underground caves, caverns, sinkholes and cities that are hiding ancient secrets.
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Explore a compilation of the biggest, most mysterious underground places and holes on Earth, including Chauvet Cave and its strange cave art; Belize's Actun Tunichil Muknal and its Mayan sacrifices; the medieval Erdstall of Europe; Turkey's large abandoned underground city Derinkuyu; and the strange megalithic underground temple of Malta, the Hypogeum of Hal-Saflieni.

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Music: "Dark Walker's Theme"
by Chill Carrier

Intro: "The Machine Thinks"
by Kevin MacLeod (incompetech.com)
Licensed under Creative Commons: By Attribution 3.0
http://creativecommons.org/licenses/by/3.0/