Vietnam Hang Son Doong Cave
Vietnam’s Hang Son Doong is a mammoth cavern so big that a Skyscraper could fit with room to spare! In fact, a half-mile block of 40-story buildings could easily fit inside the series of connected caves, making this probably the world’s biggest subterranean passage. Though the cave can be safely explored in the dry season (November to April), the rainy monsoon season brings high waters and floods the underground river, making the cave impassable.
View the gallery at: National Geographic.