Zao Onsen Dai-rotenburo

One of Japan's largest outdoor baths, tiered along a natural mountain stream and capable of holding up to 200 people at once. The water comes straight from Zao's volcanic springs and is strongly acidic at pH 1.3, which gives it a distinctive milky blue color and a sharp sulfurous smell. That acidity is the reason soap and shampoo do not work here, so it is a pure soak only. The facility splits into separate men's and women's sections, with wooden changing stalls and stone-lined pools. It is closed in winter due to heavy snowfall, making spring through autumn the window to visit.