So, in a hot springs resort, I have to find out where the hot water comes from. Fortunately, at the Merawap resort, some chap called Andrew Cullen from Shell Oil wrote a very helpful explanation in the guest book:
After finding out that the source is shallow magma at a depth of between one and two kilometres, I was able to enjoy soaking in the water.What I now don't know is why the water from hot springs is supposed to be so good for the skin. Maybe some medical expert can write that explanation in the guest book some day.