Currently the cave is about 135 miles long. The crazy thing about its length, is that it is under about 1 square mile of prairie. Not a whole lot of geographical sprawl, but intensely packed in that area!
The original hole is pretty small. It's about as big around as a basketball. It's the only known natural entrance to Wind Cave. As the cave "breathes" or adjusts to air pressure differences between the outside and inside air, the windspeed coming out of the cave can get over 35 mph!