Baths where the spring rises beneath you

In northern Okayama, Okutsuso's Kagiyu has a floor that is one continuous slab of granite with the Yoshii riverbed, and alkaline water at about 42°C (pH 9.3) bubbles up from beneath the rocks. The name comes from the Tsuyama domain lord, who locked the bath to keep villagers out. A second bath, the Tachiyu, is a natural rock hollow about 120cm deep where you bathe standing. Both swap between men and women, and two private baths are free for staying guests, 24 hours, with no booking.

The catches: the rocky floors are hard for anyone unsteady, the inn has only ten rooms and prices have risen steeply, and you'll almost certainly need a car.

At Namari Onsen in Iwate, the Shirasaru-no-yu is a hand-dug rock tub about 1.25 metres deep on average, in a two-storey-high hall, where you stand while water rises from the bottom. It's mixed, with women-only hours (6:00–7:00, 11:00–13:00 and 20:00–22:00), and all four of the inn's bathhouses are 100% free-flowing — no heating, no added water, no recirculation.

Expect about 20 steps down to the bath with no lift, no door between the changing area and the tub, and crowds from day visitors in the afternoon. The inn also has a rustic self-catering tōji wing that divides opinion.

In the mountains of Minakami, Gunma, Chōjukan's Hoshi-no-yu and Chōju-no-yu are filled by water welling up directly through the stones of the bath floor. Hoshi-no-yu, in a dim wooden hall, is mixed, with women-only time from 20:00 to 22:00; bathing garments and wrapped towels aren't allowed and there are no washing stations.

It's a deeply traditional, remote inn — expect rooms without air conditioning in parts, and plan around the mixed-bathing rules.

At Misasa Onsen in Tottori, Ohashi's half-underground Gankutsu-no-yu has three rock tubs where water wells up through what was once the riverbed: two radium springs and a thorium spring the inn says is unique in Misasa. The whole 1932 inn is a registered cultural property.

Women can use the cave bath only until 21:00, the uneven rock floor is a repeated warning for older guests, and reviewers since 2022 report uneven service.

Baths fed from below are usually small, deep and naturally hot or cool depending on the season, and they can't be heated or chlorinated, so etiquette matters: rinse before entering, keep towels out of the water, and don't expect showers or soap right beside the tub. If mixed bathing or rocky floors are a concern, every inn above also has gender-separated or private baths — our individual reports list the hours.