A blended supply source (Shin-Ubako Onsen) drawing from six wells across Motohakone and Sengokuhara. A genuinely hot natural source (61 °C at the wellhead, served to bathers at 42 °C), classified as a simple thermal spring — lightly mineralized at 478 mg/kg of evaporation residue, weakly alkaline, and gentle on the body. A faint hydrogen sulfide note carries through, and a generous 162 mg/kg of metasilicic acid leaves the kind of silica film that gives skin a noticeably smoother feel afterwards.
Good for
Aches & joints
Warmth eases stiffness and raises the pain threshold. Radon and sulfate springs show the strongest evidence for joint and muscle pain.
stiff shoulders
Smooth skin
Bicarbonate and alkalinity gently lift dead keratin while silica hydrates, leaving skin smooth. This is the basis of the beautifying-water (bijin-no-yu 美人の湯) reputation.
dry, rough skin
Recovery
Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.
fatigue, stress
Full composition
The bar is each ion's share of charge (mval%) within its group; over 20% names the spring. Gases, silica and trace metals are measured by mass only.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺40.0mgP53 national
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺25.5mgP6 national
- Magnesiummild; a laxative if you drink itMg²⁺10.7mgP63 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.94mgP78 national
- Manganesetrace; companion to iron springsMn²⁺0.29mgP46 national
- FeTotalFe0.18mgP38 national
- StrontiumSr²⁺0.10mgP17 national
- ZincZn²⁺0.02mgP46 national
- BariumBa²⁺0.0080mgP29 national
- LithiumLi⁺0.0020mgP11 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻204mgP66 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻3.7mgP6 national
- Fluoridetrace; drinking limits applyF⁻< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻< —mg
- Hydrogen phosphateHPO₄²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃162mgP74 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.2mgP21 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —mg
- CopperCu0.0020mgP42 national
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
Measured at the source
Evidence
- Maeda M. 温泉の医学的効果とその科学的根拠. J. Hot Spring Science 70:197–207, 2021.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Donaubauer AJ et al. Serial radon spa therapy on pain in musculoskeletal disorders. Frontiers in Immunology, 2024.
- Verhagen AP et al. Balneotherapy for rheumatoid arthritis. Cochrane Database of Systematic Reviews CD000518, 2015.
- Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.