Wind Cave National Park is an American national park located 10 miles north of the town of Hot Springs in western South Dakota. Established on January 3, 1903 by President Theodore Roosevelt, it was the seventh national park and the first cave to be designated a national park anywhere in the world. The cave is notable for its calcite formations known as boxwork, as well as its frostwork. Approximately 95 percent of the world's discovered boxwork formations are found in Wind Cave. The cave is recognized as the densest cave system in the world, with the greatest passage volume per cubic mile. Wind Cave is the seventh longest cave in the world with 154.2 miles of explored cave passageways and the third longest cave in the United States. Above ground, the park includes the largest remaining natural mixed-grass prairie in the United States.
The passages in the park are said to "breathe" as air continually moves into or out of them, equalizing the atmospheric pressure of the cave and the outside air. When the air pressure is higher outside the cave than inside it, air flows into the cave, raising the cave's pressure to match the outside pressure. When the air pressure inside the cave is higher than outside it, air flows out of the cave, lowering the air pressure within the cave. A large cave such as Wind Cave with only a few small openings will "breathe" more obviously than a small cave with many large openings.
There are three tours that you can take in the Wind Cave, and I took the second one which lasted about 1 hour. It was well worth the trip.