Lechuguilla Cave, in Carlsbad Caverns National Park in southeastern New Mexico, is one of the world’s longest explored caves and among the deepest in the United States, prized for its pristine condition, unusual mineral formations, and scientific importance. Once thought to be little more than entrance passages worked briefly for bat guano after a 1914 mining claim, it was dramatically expanded as a known system after cavers broke through into vast passages in 1986. Its geology is especially significant because it appears to have formed largely from sulfuric acid rising from below. Ongoing exploration, microbiological discoveries, and extreme fragility have made access tightly restricted.