As

you pass through the Chihuahuan Desert and Guadalupe Mountains of

southeastern New Mexico and west Texas—filled with prickly pear,

chollas, sotols and agaves—you might never guess there are more than

300 known caves beneath the surface. The park contains 113 of these

caves, formed when sulfuric acid dissolved the surrounding limestone,

creating some of the largest caves in North America.

Mammoth Cave National Park

preserves the cave system and a part of the Green River valley

and hilly country of south central Kentucky. This is the

world’s longest cave system, with more than 365 miles explored. Early

guide Stephen Bishop called the cave a "grand, gloomy and peculiar

place," but its vast chambers and complex labyrinths have earned its

name: Mammoth.