Reisenji Onsen

A lakeside spa town on the eastern shore of Lake Suwa, where open-air baths face the water beneath the floating castle of Takashima, a timed geyser, five sake breweries, and the great August lake fireworks.

Japan's highest onsenchi, at 1,800 m on the slopes of Mt. Shirane in Gunma. Strongly acidic milky-white sulfur waters (pH 2.5) from one of the country's most active geothermal fields, with rotenburo open to alpine views year-round.