salt's cation — pairs with chloride to make the water salty.
Bicarbonate HCO₃⁻
"soda" — softens skin, leaves it smooth.
Known good for
Aches & jointsstiff shoulders
Smooth skindry, rough skin
Recoveryfatigue, stress
Iwaya Onsen is a simple weak-radioactive spring, and at 142 Bq/kg of radon it sits squarely in the hormesis tradition that gives radon waters their following. It is a cold-mineral source (below 25 °C), so it is heated for bathing. The water is lightly mineralized and mildly alkaline — gentle and soft on the skin, colorless and clear, with a very faint hydrogen sulfide note at the wellhead. It also carries a generous load of metasilicic acid, the kind that leaves a thin moisturizing film and places it in Japan's beauty-water family.
Sampled at wellhead·Analyzed Dec 11, 2024·兵庫県立健康科学研究所·Cert no. 第5521号-1
Almost a super sento for 800 yen with radon water and a view of the Akashi Kaikyō Bridge. The place is a bit old and simple, but the magnificent view from the baths of the bridge completely makes up for it. The men's and women's zones differ quite a bit and alternate daily. The nearer one is "European style," with baths tiled with ceramic, and the farther one is "Japanese style," with baths finished in stone. Both indoor zones (spacious with high ceilings) have washing areas, a large indoor bath with a hydro-massage section, a not very hot dry sauna, and a small cold water plunge pool next to it. In the nearer outdoor zone (which was men's this time) there is a large bath with hydro-massage loungers and a utaseyu shower activated by a button. The highlight of this zone is a separate mizuburo (cold water bath) with a view of the bridge, with water around 20-25 degrees Celsius, so the contrast is small, but you can sit in it comfortably for several minutes. There are no recent photos of this zone anywhere (the mizuburo was added only last year), but it looks a bit uninteresting and worn — the baths are simply tiled, with visible rust marks on the handrails and the like. In the farther outdoor zone (which was women's) there is a beautiful (and noticeably hot) rotenburo (open-air bath) made of stone, and in it a round jacuzzi bath under a roof, where sitting reminded my wife of the Naruto whirlpools, but the water level in this bath is high, so you have to squat — not very relaxing. They have not made a mizuburo in this part (yet?), but this is aesthetically compensated by the beautiful stone finish of the baths. At first glance, the water seems simple and leaves a chlorine-like feeling, and I thought it was not onsen water, but in the dressing room there was an analysis sheet claiming it is natural radon water. There is a restaurant, with dishes mainly from seafood or local beef and local onions, prices are reasonable, the food is simple but tasty. There is a shop with tourist souvenirs at tourist prices. There is a terrace where you can sit nicely with a view of the bridge in good weather and take photos with the bridge and the concrete letters "Awaji" in the background. You can get here almost only by car (the website also mentions some local buses from other tourist spots, but it is not clear how to plan all that together on the fly). 2026-06-06 Photos from the official website correspond.
