Nuru-yu: The Gentle Baths

Marukoma sits alone on the far shore of Lake Shikotsu, reached by a single road along the water. Its signature bath, the tennen yato (天然野湯, natural wild bath), is separated from the lake by nothing but a rock ledge, one of a small number of ashimoto yushutsu (足元湧出, foot-fed) springs left in Japan, where the water rises and falls with the lake's own level. There's also a view rotenburo looking straight out over Shikotsu and the surrounding peaks, separate indoor baths for men and women, each with a sauna where you pour the water yourself, plus a cold plunge filled with lake water and loungers outside for cooling down. Two private baths can be booked as well, one with its own barrel sauna. The water is clear and faintly salty, and it's known locally for keeping you warm long after you've dried off. The inn was founded in 1915 by Sasaki Hatsutaro and is a member of Nihon Hito o Mamoru Kai (日本秘湯を守る会), the association of Japan's secluded hot-spring inns. No public electricity or water lines reach the building: it generates its own power and draws its water from the lake, running on a spring that rises naturally right at its own caldera shore.

Shinyamane Onsen Beppin no Yu features alkaline hot spring water known for its beautifying effects that leave skin feeling smooth. The facility includes a large bathhouse and sauna in a quiet setting where you can relax to the sounds of insects and birdsong. A simple restaurant and rest area make it a pleasant place to unwind throughout the day.

The headline here is the "Sumi no Yu", a bath coloured jet black by iron-rich sulfur water, one of the few genuinely black springs in Japan. Depending on the weather and season the same water can shift through milky white, grey, and pale green, which regulars call the "five-colour bath". Other pools on the property run the usual sulfurous cloudy white. The whole place has a deep-country feel, set in a narrow valley on the original Shiobara spring site.

A small, retro-feeling ryokan in Shima Onsen where every bath, indoor and outdoor, runs on real onsen water. Guests can use three private open-air baths, kashikiri (貸切), as often as they like, free and without a reservation, alongside separate indoor baths for men and women. Two room types come with their own onsen bath, and one has an onsen footbath on its terrace, looking out over the river and hills. All rooms face south onto the Shima river, with mountain views that change through the seasons. It's a small, low-key place, so the private baths rarely have much of a wait.

Shimobe Onsen has a long association with Takeda Shingen, who allegedly used these cold springs to treat battle wounds, and the alternating hot-and-cold soak that Hashimotoya still practices is said to go back to that tradition. The cold spring here comes out at around 32°C naturally, paired with a drilled hot spring at 50°C, so you move between the two. It's one of only a handful of places in Japan where you can still do this kind of therapeutic cold-mineral-spring bathing.

The senninburo at Kanaya Ryokan is, by common account, the largest all-hinoki (cypress) bath in Japan. Built in 1915 (Taisho 4), the vast wooden hall stretches roughly 20 metres and bathes you in the clean, warm scent of cypress throughout. It is a mixed bath (konyoku): women can also enter a connected women's bath and benefit from women-only hours. The ryokan itself was established in 1867 in Izu, Shizuoka, tucked in a quiet valley with spring water flowing directly from the base of Kanaya Mountain. Beyond the great hall, there is also a small outdoor rotenburo for those who want open sky. The combination of hot spring water, cypress, and a 160-year-old inn is genuinely hard to find elsewhere.

One of the best-known lukewarm baths in Izu, in the hills of Izunokuni. Everything here runs kakenagashi straight from a 38°C spring, and the tubs are held at a spread of temperatures from about 36°C up to 41°C, so you pick the one you want and stay in the cool end as long as you like. There is a waterfall bath (utaseyu, 打たせ湯) as well.

Jizo-yu is one of Kinosaki Onsen's seven sotoyu (外湯), the town's communal public baths that locals use like their own bathroom. There is a single indoor bath, deep and kept fairly hot, plus a separate shallow, lukewarm pool for small children. There is no rotenburo and no sauna here. The water is the same salty spring shared across all seven Kinosaki outer baths. Upstairs is a tatami rest area. The name comes from a statue of Jizo said to have emerged from the bath's spring source; it now stands enshrined in the courtyard, and locals traditionally bow to it before bathing. The bath grew out of one used by area villagers in the Edo period, and every August the neighborhood's Jizo Bon (地蔵盆) festival is held here, when local elders give treats to the children.

Kumenoyu is a small day-use bathhouse in Matsuyama's Kume neighborhood, close to Dogo Onsen. It has an indoor bath, a cypress-tub bath, an open-air bath, a cold plunge, and a sauna, along with three bedrock-bath rooms named for the sun, moon, and stars after the shrine next door. The spring water is smooth and faintly slippery on the skin, and it's used throughout the building, in the showers and cold bath as well as the tubs. There's also a reclining foot bath and a drinking fountain of spring water filtered through local stones. A neighborhood legend ties the spring to a great earthquake in the Ansei era: Dogo Onsen's own water is said to have stopped flowing that day, gushing out here instead near a nearby temple, until prayers sent it back to Dogo. Locals have bathed here ever since, and evenings can get crowded with regulars.

The signature bath at Fukumotoya is a semi-outdoor cave pool cut directly into the rock beside the river, low-ceilinged, dim, and fed by a radium-bearing simple spring that naturally wells up at only around 36°C. The coolish temperature and cave setting make it unlike most onsen; in winter it gets genuinely cold, but that's part of the charm of this tiny, nine-room ryokan.