More than three times the height of Niagara Falls, much of Vietnam's Mountain River Cave has remained untouched by humans. With exclusive footage, National Geographic reveals, for the first time in history, astounding evidence that this could be the largest cave in the world. Together with renowned expedition leader Howard Limbert, cave geologist Darryl Granger uncovers the formula for how the cave grew so grand. And international zoologist Anette Becher identifies unique underground species that evolved there. Finally, the team climbs to the cave's ceiling. Will their readings help confirm that they have just ascended to the top of the World's Biggest Cave?