Open-air pools in the ravine at Zaō Onsen

Zaō Onsen Dai-rotenburo

A large open-air bath built into a mountain stream, fed by some of the most strongly acidic water in Japan.

Photo: Raita Futo, Wikimedia Commons (CC BY 2.0)

Bathing: Outdoor

Zaō Onsen in Yamagata has been in use for centuries and is known for water so acidic it was traditionally credited with healing wounds. The Dai-rotenburo is a series of wooden-edged pools set directly in a wooded ravine, open from roughly late spring to late autumn.

Balnevia view

Why visit

Bathing in a forest gorge with the stream running beneath the decking is the appeal; in winter the same mountain becomes famous for its snow-covered "ice monster" trees.

The water

Thermal water

Source: Zaō Onsen spring
Mineralisation: Strongly acidic sulphur-sulphate water
Character: Among the most acidic bathing waters in Japan, at roughly pH 1.6.