1903: World's First Cave National Park

It was also the first cave anywhere in the world to be designated a national park, set aside specifically to protect the cave and its formations. The park sits about 10 miles north of the town of Hot Springs in western South Dakota.

The first recorded entry into the cave came in 1881, when American settlers went inside. A mining company explored it next and found nothing worth extracting, and the cave was later developed for tourism. When settlers arrived, the Lakota and the Cheyenne already lived in the area. For the Lakota, the cave is the place where humanity emerged in their creation story, and they call it Maka Oniye, which means breathing earth. Because the entrance is so small, there are no known records of Native Americans going inside.

Air moves through the cave from its entrance, which makes it one of the most famous breathing caves anywhere. Measured by how much passage fits into each cubic mile, its network is the most tightly packed of any known cave. Boxwork is the cave's signature feature. Calcite grew into thin blades, and as the surrounding limestone wore away, those blades were left standing in a honeycomb-like grid. About 95 percent of the boxwork discovered anywhere in the world lies inside this one cave.

Most visitors come for the cave tours, yet the land above ground holds plenty more. Above ground, the park protects a broad swath of mixed-grass prairie that ranks among the nation's biggest. Visitors often spot bison, elk, prairie dogs, pronghorns and coyotes roaming the grass. Hikers can follow routes such as the Lookout Point Trail through the grass and hills above the cave.