Thunderhead Underground Falls is a smaller cascade waterfall in South Dakota, with a drop of about 30 ft.
Thunderhead Underground Falls sits on Rapid Creek (subterranean, via former gold mine tunnel). You'll find it in Pennington, South Dakota. It is protected within Private (privately operated tourist attraction).
This is a short hike waterfall: reached by a short trail, generally under about a mile each way. Base a visit out of Rapid City, the closest town.
When to go: Year-round (underground). Like most waterfalls, Thunderhead Underground Falls runs highest after snowmelt or heavy rain and can thin out in late summer, so it's worth checking recent streamflow and the land manager's conditions page before a long drive.
