Thank you Vivi.
You stayed in Jettegrytene - Giant Kettles - Nissedal, Telemark, Norway.
I found a lot of Flickr and Panoramio pictures but not a lot of other information. I do know how the pot-holes were formed, but I still don't know how the water temperature changes with the seasons. I could not find the pictures of the bathers in the foaming water, but I suspect the bubles are created by a small waterfall, like this
Next, I do hope there is going to be a renewed interest in this thread...and that it is easier than the last two...
