Kawarage Oyutaki (Hot Water Waterfall)

A hot spring waterfall, the onsen water itself cascades about 20 meters down a rocky gorge, and the pool at the base is warm enough to soak in. The spring is strongly acidic (pH 1.41), one of only a handful this intense in the world, so the water has a slight sting on eyes and sensitive skin. Open only in summer (roughly July to late September), and you reach it by a 15-minute walk through the sulphurous landscape of Kawarage. Swimwear required; no soap.