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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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This National Park Is Home to the Longest Cave in the World—and Only 426 Miles Have Been Explored So Far

Every single one of America’s national parks is unique, with a beauty that’s entirely its own. “The park's cave system is the longest in the world, stretching over 426 miles, and offers visitors a ...