I would like to introduce Renge Onsen, located at the trailhead of Mount Shirouma, one of Japan's 100 Famous Mountains.
Renge Onsen is the ultimate hidden hot spring, located at an elevation of 1,457 meters at the Niigata-side trailhead of Mount Shirouma in the Northern Alps, one of Japan's 100 Famous Mountains.
The highlights of Renge Onsen are its four outdoor open-air baths, scattered along a mountain path from the lodge. From these baths, you can enjoy a 360-degree panoramic view of the Northern Alps! For the best experience, it's recommended to visit on a clear day.
The onsen is open from late June to late October, and during this period, you can also enjoy day-use bathing.
However, during the peak hiking and autumn foliage season from late July to mid-October, there is a bus service from Itoigawa Station. But at the beginning of the season, the road is closed, so you must hike approximately 8.5 kilometers from Hiwa-daira to reach the onsen.
Accommodation is available only at the Hakubadake Renge Onsen Lodge. The building is a sturdy reinforced concrete structure, but it is essentially treated as a mountain hut, so accommodations are typically in shared rooms (private rooms are available for an additional 4,000 yen).
cited from https://niigata-kankou.or.jp/spot/10641
The charm of Renge Onsen lies in the opportunity to soak in the hot springs while enjoying such breathtaking scenery.
If you have the chance to visit Japan, be sure to check it out!