Iiyama Onsen

On the limited express Romance Car out of Shinjuku, the city dissolves surprisingly quickly. Within forty-five minutes the Odakyu line is moving through forested hills in Kanagawa Prefecture, and the valley that holds Iiyama Onsen sits in the Atsugi mountains above a murmuring tributary stream. There is no resort town here, no grid of souvenir shops or illuminated hotel frontages. There is one old wooden ryokan called Motoyu, built above the stream, where the water coming out of the ground carries a pH of 11.3.

That number is worth pausing on. A pH of 7 is neutral. Most alkaline springs in Japan sit between 8 and 9.5. At 11.3, Iiyama Onsen ranks among the highest-alkaline spring waters recorded in the entire country. When you lower yourself into a bath here, the feeling is not simply warm water. The water has a distinctive slip to it, almost frictionless against the skin, and it has been described since the Edo period as a beauty spring because strong alkalinity acts as a natural exfoliant, dissolving the outer layer of dead skin cells as you soak. After twenty minutes you step out and your skin feels faintly different, not just clean but smooth in a way that tends to last through the following day.

The indoor bath at Motoyu is lined with old wood and built low to the ground, in the style of baths that have not been redesigned to look rustic but simply are. The open-air bath looks directly onto the mountain stream below, and depending on the season you are listening to either rushing snowmelt or the slower summer current. A private kawa-buro style option lets you sit at the water level with the river moving past just below the wooden deck railing, which is a strange and genuinely good feeling.

The kitchen here is taken seriously. The head chef holds a Shijo Shinryu title in the traditional Japanese culinary arts, and kaiseki meals use local Kanagawa ingredients through the seasons. The freshwater ayu fish from the stream, served in autumn, is a specific reason some guests return. But the central draw of Iiyama remains the water, which is plain-looking and clear and almost inert-seeming until you have been in it for ten minutes and your skin starts to feel like it belongs to someone who sleeps eight hours a night.