Douko no yu (Wild Hot Spring Onsen)
Hot Spring Overview
Copper Powder Bath is a wild onsen located in the Manza Onsen area of Tsumagoi Village, Gunma Prefecture, and is renowned as one of Japan’s most sulfur-rich, strongly acidic hydrogen-sulfide springs. True to its name, the yuno-hana (mineral crystals) are pale yellow and resemble a half-cooked omelette. The water temperature ranges from about 35–42 °C, depending on the spot. It’s a completely wild bath using natural stream beds and waterfall pools, free and open 24 hours a day.
Hot Spring Details
Practical Info
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Facilities
Access Information
From JR Agatsuma Line’s Manza-Kazawaguchi Station, take the bus toward Manza Onsen (about 40 min).
From Manza Pass, descend the valley stream for roughly 30–40 min, or approach via the Manza Ski Resort route.
Neither path is marked; both require bushwhacking and stream-walking, making the hike highly challenging.
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About Secluded Rating
Access (Difficulty to Reach)
Over 1 hour walk
Scenery & Atmosphere
Hidden wilderness
Onsen Value
Unique spring
With its untamed riverside setting, total absence of changing rooms or toilets, and no on-site attendants, this onsen fully satisfies the criteria for a “super secret spring” (“chō-hitō”). The unique pale-yellow yuno-hana, likened to a soft-boiled omelette, only adds to its hidden-gem appeal.