The story of Carlsbad Caverns begins 250 million years ago in the Permian period, when the Capitan Reef grew along the edge of an inland sea. After the sea evaporated and the reef was buried under thousands of feet of sediment, the story might have ended there - except for what lay even deeper. The Delaware Basin beneath the Guadalupe Mountains contains substantial petroleum reserves. As these hydrocarbons matured over geologic time, they released hydrogen sulfide gas that migrated upward through cracks in the overlying rock. When the H2S encountered oxygen-rich groundwater in the limestone, it formed sulfuric acid - and sulfuric acid dissolves limestone aggressively. The cave's enormous chambers, its maze-like passages, and its distinctive gypsum deposits (a byproduct of the limestone-acid reaction) all testify to this unusual origin. Most caves are carved from the top down by descending water; Carlsbad was eaten from the inside out by rising acid.

In 1898, a sixteen-year-old cowboy named Jim White was riding near the escarpment when he noticed what looked like a plume of smoke rising from the ground. Investigating, he discovered not smoke but bats - hundreds of thousands of them spiraling out of a hole in the earth. White returned with a homemade ladder fashioned from wire and wooden slats, lowering himself into the darkness to explore what he found below. Over the next two decades, he named the cave's major features with a mixture of wonder and whimsy: the Big Room, the Kings Palace, the Queens Chamber, the Bottomless Pit, the Rock of Ages, the Witch's Finger. He led tours for curious visitors, lowering them into the cave in a guano mining bucket. His persistent advocacy brought the cave to national attention, and in 1923 President Calvin Coolidge declared it Carlsbad Cave National Monument. Full national park status followed in 1930. The town of Carlsbad, which gave its name to the cavern, was itself named after the Bohemian spa town Karlsbad (now Karlovy Vary in the Czech Republic), both names meaning 'Charles's Baths.'

Visitors today can enter Carlsbad Cavern through the Natural Entrance, following a steep switchback trail that descends 750 feet through the cave's maw - the same route Jim White took with his wire ladder, now paved and lit. Alternatively, elevators installed in 1932 drop visitors directly into the heart of the cave in about a minute. Either way, the destination is the Big Room, a single chamber so vast that the Statue of Liberty could stand upright with room to spare. The trail loops for 1.25 miles through a forest of formations: stalagmites rising from the floor, stalactites hanging from the ceiling, delicate soda straws, massive flowstone draperies, and towering columns where the two have merged. The temperature holds steady at 56 degrees Fahrenheit year-round, a reminder that you're standing in a pocket of ancient air beneath 750 feet of rock. Exploration continues: in 2013, park cavers discovered 'Halloween Hall,' the largest new area found in Carlsbad Cavern in more than 25 years.

Every evening from April through October, Brazilian free-tailed bats pour out of the Natural Entrance in a spiraling vortex that can take hours to complete. The colony, which varies from several hundred thousand to over a million individuals depending on the year, roosts in a section of the cave closed to visitors. At dusk, they emerge to hunt insects across the Chihuahuan Desert, consuming several tons of moths, beetles, and other prey each night before returning at dawn. The spectacle has been a draw since Jim White's time - he called it 'the most wonderful sight I ever witnessed.' The park maintains a bat flight amphitheater where rangers provide evening programs, though no artificial lighting or flash photography is permitted to avoid disturbing the colony. The bats' tenancy of Carlsbad Cavern extends back at least 5,000 years, based on the depth of guano deposits - the same guano that was commercially mined in the early 20th century and that first brought the cave to wider attention.

Carlsbad Cavern is just one of over 100 known caves within the national park's boundaries. The most significant is Lechuguilla Cave, discovered in 1986 when cavers broke through into passages that had been sealed for millions of years. Lechuguilla extends more than 150 miles of surveyed passage and descends 1,604 feet, making it the deepest limestone cave in the continental United States. Its isolation preserved formations of extraordinary delicacy: gypsum chandeliers, cave pearls, and rare minerals that exist in few other places on Earth. The cave is closed to recreational visitors; entry requires special permission for scientific research only. Studies of Lechuguilla's isolated microbial ecosystems have yielded bacteria with natural resistance to modern antibiotics - organisms that evolved their defenses millions of years before humans invented the drugs. The caves of Carlsbad Caverns National Park continue to reveal secrets, reminders that the world beneath our feet remains largely unexplored.