A cave born from acid
Lechuguilla Cave's existence contradicts the typical story of cave formation. Most of the world's limestone caves are epigenic, formed by carbonic acid in rainwater seeping down from the surface and slowly dissolving the rock. Lechuguilla is a hypogenic cave, created by a far more aggressive process that started deep within the earth. The cave lies within the Guadalupe Mountains, which are composed of an ancient Permian-age reef. Below this limestone are oil and gas deposits that release hydrogen sulfide (H₂S). This gas migrated upward through fractures and mixed with oxygenated groundwater.
The resulting chemical reaction produced sulfuric acid (H₂SO₄). This powerful acid dissolved the limestone (calcium carbonate) at an accelerated rate, carving out the vast passages from the bottom up. This process of sulfuric acid speleogenesis is responsible for the cave's immense size and the unique collection of minerals found within its passages. The cave is currently mapped to over 152 miles (244 kilometers) in length and reaches a depth of 1,604 feet (489 meters), making it the second deepest limestone cave in the United States and one of the longest in the world. Argon-argon dating of alunite, a mineral byproduct of the acid reaction, suggests the cave is at least four million years old.