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
Akashi Kaikyo view, Matsuho-no-sato, Awaji, Hyogo
Almost a super-sento, ¥800, with radon water and a view of the Akashi Kaikyo Bridge. The place is a bit old and plain, but the great view of the bridge from the baths makes up for everything.
The men’s and women’s areas differ quite a lot, and they swap by day. The closer one is in a “European style”, with tiled baths, while the farther one is in a “Japanese style”, with stone-lined baths.
Both indoor areas, which are spacious and have high ceilings, have washing areas, a large indoor bath with a hydro-massage section, a not-too-hot dry sauna, and a small cold-water tub next to it.
In the closer outdoor area, which was the men’s side this time, there is a large bath with hydro-massage loungers and an utaseyu shower that turns on with a button. The feature of this area is a separate mizuburo with a view of the bridge, filled with cool water, probably around 20-25 degrees, so the contrast is not very strong, but you can sit in it comfortably for a few minutes. There are no current photos of this area anywhere, since the mizuburo was added only last year, but it looks a little unremarkable and worn, with the baths simply tiled and signs of rust visible on the handrails and so on.
In the farther outdoor area, which was the women’s side, there is a nice, and noticeably hot, rotenburo made of stone, and inside it a round jacuzzi bath under a roof. Sitting in it reminded my wife of the Naruto whirlpools, but the water level in this bath is high, so you have to crouch, which makes it hard to relax. They have not made a mizuburo in this part yet, but that is offset by the aesthetically pleasing stone finish of the baths.
At first glance the water seems ordinary and leaves a chlorine-like feeling, and I was already thinking it might not be real onsen water, but in the changing room I found an analysis sheet stating that it is natural radon water.
There is a restaurant, with dishes mostly based on seafood or local beef and local onions, the prices are reasonable, and the food is simple but tasty. There is also a shop with tourist souvenirs at tourist prices. There is a terrace where it is great to sit with a view of the bridge in good weather, and to take photos with the bridge and the concrete Awaji letters 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 immediately clear how to plan all of that together.
2026-06-06 Photos from the official website, and they match.
