It may be difficult to think of caves as spacious and grand, but these are no ordinary caves. Humans first discovered this complex limestone labyrinth some 4,000 years ago beneath the Green River Valley of Edmonson County, Kentucky. To this day, explorers have yet to find an end to it, despite mapping more than 400 miles of passageways. It is by far the longest such system on Earth, featuring the richest known habitat for cave wildlife in the world.


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