Akayu Onsen Yamaguchikan

Akayu Onsen Yamaguchikan is a lone mountain inn set on the slopes of Mount Naeba, one of Japan's 100 Famous Mountains, at roughly 1,060 meters above sea level in Yuzawa, Niigata Prefecture. No road reaches it: the only way in is a hike of about two hours along a mountain trail beside the Akayu-zawa stream. Said to have opened around 1897, it has no electricity and is lit by oil lamps, and has long been cherished by hikers and secret-hot-spring enthusiasts.

The water is a sodium-calcium chloride spring (the Ao-yu bath being a chloride-sulfate type). Source temperatures exceed 50C, and the greatest draw is that the water wells up naturally through the pebbles of the riverbed and is enjoyed exactly as it emerges, with no heating, dilution, recirculation, or chlorination. Each bath differs in color, scent, and feel: the Tamago-yu is a cloudy reddish-brown with a metallic note, while the Ao-yu is a clear pale blue.

Three open-air baths line the riverbank: Tamago-yu, Ao-yu, and Yakushi-yu, all essentially mixed-gender with scheduled men's and women's times. Day-use bathing is 500 yen per adult (9:00-15:00), and an overnight stay with two meals starts at 9,000 yen. The inn operates from late April to early November; in winter both the forest road and trail are closed and the inn cannot be reached.