Gather your courage and take a trip deep into the dark recesses of some of the world's most unusual caves. It's pitch black, toxic gases bubble up from deep below, sulfuric acid drips from above, and carbon monoxide fills the air. It's not a scene from a horror movie, but the interior of a startlingly beautiful--but treacherous--cave. Today, scientists are penetrating the depths of incredible caverns like this one, discovering abundant life hidden in the dark, and shedding new light on how life may have begun on earth and may exist on Mars and other worlds. NOVA joins top scientists as they explore some of the most exotic and dangerous caves in the world, off limits to all except researchers. Explore Cueva de Villa Luz in Mexico, an active and dangerous cavern that teems with life despite the dark and noxious environment. Travel to Carlsbad Caverns and learn how this world-famous tourist attraction concealed an astonishing scientific secret. And squeeze through the deepest cave in the Americas, New Mexico's Lechuguilla, home to the world's most stunning and fragile cave formations.