One Of The World's Longest Caves Is A Hidden South Dakota Crystal-Filled Monument That Literally Dazzles

South Dakota may conjure up images of Mount Rushmore, sweeping prairies, and the rugged beauty of the Badlands, but it's also a destination filled with surprises. The Black Hills region, a lush expanse of pine-covered peaks and ancient rock formations, is brimming with hidden treasures. Some of its features include South Dakota's "best secluded swimming hole" and a secret canyon trail with enchanting scenery. However, the wonders of this region extend far beyond the surface — they also run deep underground. West of the town of Custer lies a literal gem of a destination: Jewel Cave National Monument. 

Named for the shimmering calcite crystals that adorn its chambers, this underground marvel stretches at least 220 miles, making it one of the longest cave systems in the world and the second longest in the U.S., according to History. New passageways are uncovered each year, with around 3 miles of new terrain charted annually. While parts of the cave are off-limits for research and safety, visitors can explore its wonders through a ranger-led tour. Whether you're a casual observer or a seasoned spelunker, Jewel Cave offers a crystalline adventure unlike any other.

Frank and Albert Michaud discovered Jewel Cave in 1900 and initially hoped to mine its sparkling crystals. When the gems proved to be of little value, the brothers transformed the cave into a tourist attraction, complete with a Jewel Cave Dancing Club. In 1908, President Theodore Roosevelt designated the site a national monument, though at the time, only 2 miles of the cave were known. It wasn't until 50 years later that the cave's immense scale came into focus. Explorers Jan and Herb Conn spent two decades mapping 65 miles of passageways, setting the stage for its designation as one of the world's longest caves.

Today, visitors can experience Jewel Cave through four unique tours. The Discovery Tour is perfect for beginners, offering a 20-minute look at one of the cave's crystal-filled grand chambers. For those ready to stretch their legs, the Scenic Tour is an 80-minute walking journey showcasing honeycomb-like boxwork formations. History buffs will enjoy the Historic Lantern Tour, which takes participants along a challenging rocky path and some narrow passages. For the truly adventurous, the Wild Caving Tour provides a heart-pounding, three- to four-hour experience involving belly crawls, chimney climbs, and squeezing through spaces barely larger than a backpack. For this tour, participants must be at least 16 years old.

It's best to visit Jewel Cave in late spring or early fall when the weather aboveground is mild, but remember to pack layers since the cave stays chilly year-round. You may encounter bats or snakes in or around the cave, but they shouldn't bother you as long as you don't touch or approach them. The surrounding park offers picnic spots, a visitor center, and nature trails, including the short Roof Trail and the longer Hell Canyon Trail.

Jewel Cave owes its dazzling formations to a process that began 40 million years ago. Rainwater seeped into the ground, mixing with carbon dioxide to create a solution that dissolved the limestone beneath the Black Hills. As the acidic water drained away, it left behind calcite deposits that crystallized into the glittering features that gave the cave its name. In some places, the translucent white crystals, known as dogtooth and nailhead spar, are layered as thick as 6 inches and sparkle when hit with light.

Beyond the crystals, Jewel Cave is home to a variety of otherworldly formations. Helictites twist and spiral like delicate, frozen tendrils. Scintillites sparkle with a sugary glimmer, while frostwork formations resemble icy feathers frozen mid-flight. Meanwhile, hydromagnesite balloons have a silvery, inflated appearance, adding to the surreal beauty. During winter, the cave becomes a sanctuary for seven bat species, making it one of the largest hibernation sites in the U.S. After your visit to Jewel Cave, take a thrilling road trip to see some of South Dakota's other natural wonders on the Needles Highway.