Zao Onsen

Zao Onsen se encuentra a unos 880 metros de altitud en las laderas occidentales del macizo volcanico Zao, en la ciudad de Yamagata. El manantial es uno de los onsen mas acidos de Japon, con un pH de entre 1,2 y 1,8, suficientemente acido para corroer joyas metalicas. El agua es del tipo sulfuroso de sulfuro de hidrogeno, blanca lechosa, con un intenso olor a azufre. En invierno, el fenomeno juhyo transforma los arboles de las pistas de esqui en figuras esculturicas de hielo conocidas como monstruos de nieve, uno de los espectaculos naturales mas fotografiados del Tohoku. National Geographic selecciono Yamagata — y Zao en particular — como uno de sus destinos Best of the World 2026.

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.

Zao Onsenは日本・Yamagata Prefecture, Tohokuに位置する硫黄泉・強酸性泉。源泉温度は55℃、標高880mの地に湧出する。泉水は強酸性（pH約1.5）の泉質、Sulfuric Acid・アルミニウムを豊富に含む、総溶解固形物3200 mg/Lが特徴だ。720年頃から利用されてきた。皮膚科治療・リウマチ療法・慢性皮膚疾患治療など多様な温浴療法が行われる。施設では露天風呂・大型公衆浴場・旅館滞在などが体験できる。