The water here is not hot. That is the first thing to understand. At Masutomi, in the mountains of Yamanashi, the tradition is to sit in lukewarm baths for a long time — not the brisk plunge and retreat of more famous onsen, but something closer to simply being still in water. The springs carry an exceptionally high radium content, and for generations people have come not for spectacle but for healing: diabetes, rheumatism, the slow accumulations of ordinary life. This is a tōji-ba, a place where bathing is treatment, where days are structured around immersion rather than excursion. You lower yourself in and you stay.

Eight ryokan are scattered along a gorge carved by the Hontani River, within the bounds of Chichibu-Tama-Kai National Park. The peaks of Kinpu-san and Mizugaki-yama rise beyond the valley walls. There is a legend connecting these waters to Takeda Shingen, as there often is in Yamanashi — the warlord's hidden bath, kept secret for his troops. What feels more verifiable is the presence of literary figures: Takahama Kyoshi, Ibuse Masuji, writers drawn perhaps not only by the efficacy of the waters but by the particular quiet of a place that asks you to do very little. The village that once bore the name Masutomi existed from 1875 to 1959, and something of that modest, self-contained world persists in the settlement that remains.

To stay here for several nights would be to adopt a different rhythm. You would wake, bathe slowly, walk along the gorge, return, bathe again. The water is cool enough that the body does not protest long immersion; the mind, eventually, follows. There is no urgency. The national government designated Masutomi a health-resort onsen in 1965, an official acknowledgment of what the place had already been for a very long time. It is not a destination that explains itself quickly, and that is rather the point.