[ Black Hills Photo Gallery - Central Hills - Thunderhead Underground Falls] Located inside the oldest gold mine open to the public in the Black Hills is Thunderhead Underground Falls. It is an easy 600 foot walk inside the cavern. Thunderhead Underground Falls is located ten miles west of Rapid City on highway 44. Once the stream exits outside, it exits into this creek. The waterfall created by the stream exiting outside. The view as you enter the mine. The stream runs along the right side. The underground falls themselves. The walls of the mine near the falls. The view walking back towards the mine opening. More colors on the mine walls. The sign says "Traces of Copper." The top of the falls at the mouth of the mine.