- [Narrator] Wind Cave is a very complicated three-dimensional geological maze.

Airflow at the entrance indicates only 2% of the total volume of the cave has been discovered, indicating there is a lot more to be found.

The origin of Wind Cave geology goes back about 330 million years ago when sea levels covered the central portion of North America, from the Arctic to the Gulf of Mexico.

Within the sea lived many animals covered with shells made of calcium carbonate.

Over the millennium, the marine animals died.

Their shells and other debris fell to the sea floor to become limestone.

Soon after a layer of several hundred feet thick was deposited, the seas receded for a time, and the limestone dissolved by rain and groundwater forming caves and sink holes.

When water seeps into the ground, it picks up a great deal of carbon dioxide, making a weak carbonic acid.

As soon as the carbonic acid encounters limestone, it begins to dissolve.

Now the seas returned and deposited another layer of red sediment to fill the caves and cover the limestone bed.

Beginning about 65 million years ago, near the end of the Cretaceous period, great heat under southwestern South Dakota and Eastern Wyoming pushed rock upward under the sedimentary rock.

Under enormous pressure, the rock layers bent, fractured, stretched, and bubbled up to form a large dome as much as 14,000 feet high.

Once the limestone was re-exposed to both surface and underground water, a carbonic acid was formed to redevelop the caves.