Natural History

North America’s Top 10 Natural Wonders

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David Muench/CORBIS

Mammoth Cave

This underground National Park preserves what one early visitor described as a "grand, gloomy and peculiar place" that's a part of the Green River valley and hilly country of south central Kentucky. It is by far the world's longest known cave system, with more than 400 miles (644km) explored.

Humans have been venturing into its vast chambers and complex labyrinths for at least 6,000 years and in 1981 it was named as a World Heritage Site. (Source: National Park Service)

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