There's over a hundred miles of cave in Wind Cave National Park, but there's only one little opening to the surface. So when the atmospheric pressure changes on the surface, it takes quite a while for the atmospheric pressure inside the cave to equilibrate with the pressure outside. When I went to this cave earlier today, the atmospheric pressure on the surface was lower than the pressure inside the cave, and there was a quite strong wind coming out of the little hole that leads into the cave. The ranger held up a ribbon. It was blowing strongly in the wind coming out of the cave. She estimated the wind at around 25 mph. The somewhat heart shaped hole in the center of the photo is the natural entrance to the cave. Most other caves in the world have lots of connections with the surface so the air doesn't have to move as fast in order to equilibrate with the changing surface air pressure.
There's beautiful countryside in the area of the cave. It's located where the great plains meet the black hills of South Dakota.
Here are a few photos from inside the cave.