Natural Entrance for Wind Cave
Wind Cave is the first cave national park and currently, the 6th most extensive cave network in the world. It's most famous for its large concentration of a fairly rare cave formation known as boxwork. Boxwork is formed when dissolved calcium carbonate crystallized in cracks in surrounding rock. The surrounding rock then erodes away and leaves the boxlike calcite crystals.
This image shows the natural entrance to Wind Cave. Wind Cave earned its name due to strong wind currents that emit from the natural entrance based on differential barometric pressure.
The Lakota tribe includes the natural entrance of Wind Cave in their creation stories. According to the Lakota, the buffalo emerged from the natural entrance to the Black Hills, and the Lakota followed them.