Hidden deep inside Vietnam’s Phong Nha–Kẻ Bàng National Park is Sơn Đoòng, the largest cave on the planet. It is so immense that it generates its own weather. Within this vast underground world, clouds form, rivers carve their paths, and dense forests grow beneath openings in the cave ceiling. First discovered in 1991 and fully explored only in 2009, its enormous chambers rise more than 200 meters high—large enough to hold a 40-story skyscraper. Sunlight pours through collapsed sections of the r