Yatsugatake Honzawa Hot Spring

Honzawa Onsen is a mountain hut deep in the Yatsugatake (八ヶ岳) range, reachable only on foot, an hour and a half to three hours' hike from the nearest trailhead parking, with no road to the door. The reward is an open-air bath hikers call Unjo no yu (雲上の湯), the bath above the clouds, often cited as Japan's highest-elevation outdoor bath. The water is cloudy and runs quite hot, with views out toward Io-dake and the surrounding peaks while you soak. Bathing there is mixed-gender, some people in swimwear, some not, and there's nowhere to change, just a few planks laid down at the water's edge. A separate indoor bath back at the lodge draws from a different source. Day hikers can use the outdoor bath without staying the night. It's a rustic mountain hut, not a spa, so bring your own towel.