The Alien Underground

Half a mile below the surface of the earth, in a cave too hot to explore without an ice-packed suit, NASA scientist and Nat Geo explorer Penny Boston clambers around glassy crystals that are taller than telephone poles and wider than dinner tables. But it's not The Crystal Cave's grandeur she's interested in -- it's what may be hibernating inside the crystals. Astrobiologists like Penny Boston scour the Earth's most hostile environments for microorganisms, to see if they hold clues to what life might look like on other planets - maybe even planets in our solar system. For more information on this episode, visit nationalgeographic.com/overheard