American adventurer Megan Ross stands on a moss-coated spiral stalagmite in Hang Son Doong, Vietnam, the biggest known cave in the world. This section of the enormous cave is called Watch out for Dinosaurs and it is the sort of place one might expect to see a Pterodactyl wing in from the rising cave mist. More than 100 meters above Megan is a gaping sky hole, a section of the cave ceiling that collapsed eons ago. Visitors to this cave invariable feel awed and humbled by the cave's immensity.