At the base of active volcano Iwaonupuri, at 750 metres elevation, Goshiki Onsen delivers the kind of raw mountain bathing experience that hot spring purists dream about. The outdoor baths look out over soaring alpine scenery, and the sulphur-rich waters are so acidic — pH 2.6 — that you'll feel them tingling on your skin almost immediately. The attached ryokan is deeply rustic, adding to the sense of having stumbled on something wild and authentic. You'll need a car to reach it, which only adds to the feeling of earned reward.