Trümmelbach Falls

Taken right from Wikipedia

Trümmelbach Falls (German: Trümmelbachfälle) [...] are a series of ten glacier-fed waterfalls inside the mountain made accessible by a tunnel-funicular, built 1913, stairs, and illumination.

To try to add more commentary to that, this waterfall seems to have been formed when water found a soft part of the inside of a mountain. Making most of the falls hidden from human view. Someone drilled a hole out of the mountain to create pedestrian walkways and a series of stairs to observe the falls first hand. It was amazing how we were in 80+ degree weather, then would walk 10 yards inside the caves and suddenly be in refrigerator temperatures. It changed quite quickly.

It wasn't a small waterfall either, as you can tell from the videos, it was quite powerful.Read more