Mammoth Cave
Mammoth Cave is the worlds largest network of underground passages at over 550km. It includes over 200 species of fauna that live within it, including endemic and endangered species. The surface features are notable as well with Big Woods representing the old growth hickory and oak from the Eastern U.S. In addition to it's World Heritage Status the area is also an UNESCO Biosphere.
Location
United States
37° 11' 36.888" N, 86° 9' 29.0916" W
US