Zao Onsen

Zao Onsen sits at approximately 880 metres altitude on the western slopes of the Zao volcanic massif in Yamagata City, Yamagata Prefecture, in the Tohoku region of Honshu. The spring is one of the most strongly acidic onsen in Japan, with a pH regularly measured between 1.2 and 1.8 — a level acidic enough to corrode metal jewellery and irritate sensitive skin after prolonged immersion. The water is a sulphurous hydrogen sulfide type, milky white in colour, with a powerful rotten-egg odour and a characteristic sharp sensation on the skin. This extreme acidity, produced by volcanic gases dissolving directly into the groundwater from the active Zao caldera above, gives the water recognised antiseptic and dermatological properties in traditional Japanese bathing culture. Zao's history as a spa dates to approximately 720 CE, when the monk Gyoki is said to have discovered the spring. The resort today combines the onsen tradition with year-round mountain activities: in winter, the slopes produce the Zao juhyo phenomenon — trees encased in ice and snow, creating towering sculptural forms known as 'snow monsters' that have become one of Tohoku's most photographed natural spectacles. In 2026, National Geographic selected Yamagata Prefecture, and Zao in particular, as one of its Best of the World destinations. Multiple public outdoor baths (rotemburo) and large ryokan facilities operate in the resort town.