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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.