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Vietnam's Sơn Đoòng Cave is the largest known cave passage in the world. It is so enormous that clouds can form inside it because of differences in temperature and humidity. The cave even contains its own jungle and flowing river. Some sections are tall enough to fit a 40-story skyscraper.
Scientists continue to discover new species living inside. It feels less like a cave and more like a hidden world beneath the Earth.
© National Geographic