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Hang Son Doong, a cave in Vietnam, is big enough in places to accommodate a New York City block of skyscrapers. The cave in the Annamite Mountains contains a river and jungle (its name translates to "mountain river cave") and even its own thin clouds, and its end remains out of sight. It's part of a network of about 150 caves in central Vietnam near the Laotian border.
This beautiful cave in Vietnam contains a jungle, is big enough to house a skyscraper, and was only discovered in 2009.
Mammoth Cave National Park preserves the cave system and a part of the Green River valley and hilly country of south central Kentucky. This is the world's longest known cave system, with more than 390 miles explored. Early guide Stephen Bishop called the cave a "grand, gloomy and peculiar place," but its vast chambers and complex labyrinths have earned its name - Mammoth.