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This National Park Has the Longest-known Cave System in the World—With Over 400 Miles of Passages and a Frozen Waterfall

Along with underground adventures, Mammoth Cave National Park in Kentucky offers great hiking and kayaking, too.
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From Hiking, Biking and Cave Tours: Top 11 Things to Do Mammoth Cave National Park in Kentucky

Located in Central Kentucky, Mammoth Cave National Park is a UNESCO World Heritage Site and an International Biosphere Reserve. It is home to the longest known cave in the world and features over 400 ...