One unlucky day 13,000 years ago, a slight, malnourished teenager missed her footing and tumbled to the bottom of a 100-foot pit deep inside a cave in Mexico's Yucatán. Rising seas flooded the cave and cut it off from the outside world-until a team of divers chanced upon her nearly complete skeleton in 2007. Intricate detective work reveals that the young woman's bones are among the earliest known human remains in the Americas. What drove her to venture nearly a mile underground inside a vast cave? Where did her people come from, and why does she look so distinct from today's Native Americans? From a stunning Mexico cave to the wilderness of the Yukon, from the genetics lab to the forefront of forensics, NOVA pursues tantalizing new clues that are rewriting the story of the forgotten first people who ventured into our continent.