Embark on an extraordinary journey into the underground world of caves. Beneath our sunlit world lies an unknown universe of breathtaking caverns, treacherous passages and absolute darkness. Join a National Geographic expedition to document the splendours of the world's most exotic new cave for the first time. Relive the excitement of those cavers who, in 1986, first entered Lechuguilla, New Mexico. Marvel at the pristine beauty of gypsum chandeliers, gnarled calcite columns and jewel-like lakes. Then, join the 1972 expedition that stumbled upon a passage connecting the Mammoth and Flint Ridge cave systems, creating the longest cave system in the world. And relive the thrilling discoveries and heartbreaking tragedies of caving history.