Yamagata Zao Rotenburo

High on the flanks of Mount Zao, straddling the border between Yamagata and Miyagi prefectures in Tohoku, the Zao onsen district is best known in winter for its extraordinary snow-laden trees — the juhyo, or ice monsters — but the open-air rotenburo here offer a bathing experience of equal intensity across all seasons. The volcanic crater lake nearby and the wild alpine landscape set a stage of stark, elemental beauty that few Japanese spa destinations rival.

Sulfurous water emerges at a skin-warming 39 °C, its notably acidic pH of 4.3 giving it a lively, tingling sensation on contact. The acidity also imparts the water's characteristic pale milky-green colour, one of the most visually distinctive in Japan's onsen repertoire. Bathing in the stone-lined outdoor pools while mountain mist rolls down the slopes, particularly in autumn when the surrounding beech forests ignite with colour, represents one of the great sensory experiences of Japanese onsen culture.