Bizarre creatures, poisonous gases and primitive life forms await on a journey into spectacular underground caverns. Could they explain how life on earth began almost four-billion years ago? Destinations include Lechuguilla Canyon - mapped for more than 100 miles and home to places such as "Freak-out Traverse", "Death Pit" and "Chasm Drop". Producers Jackie Higgins and Sarah Holt ; Editor Matthew Barrett
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