Photos: studio IRONY · CC BY 3.0 · iloverjoa · CC BY 3.0 · そらみみ · CC BY-SA 4.0 · 663highland · CC BY-SA 4.0 (4, 5)
Arima Onsen, one of Japan's three oldest hot springs, has two distinct waters in one small town: kinsen, an iron-rich sodium-chloride brine that comes up clear and oxidises to rust brown, and ginsen, a clear carbonated and radium water. The two public baths, Kin no Yu and Gin no Yu, serve one each.
Why it's unique
The claim is the pairing: two contrasting spring waters in one small town, and no volcano nearby, which Kobe City notes is rare for springs this hot. It is a cautious claim, and a reviewer may judge Arima a famous good afternoon rather than a singular one.
What you can do here is bathe in both in an afternoon, a few minutes' walk apart, and see the spring heads steaming in between.
Before you go
Do both public baths with the two-bath ticket, and do Kin no Yu second: it is the hotter, saltier water. Kin no Yu closes on the 2nd and 4th Tuesdays, Gin no Yu on the 1st and 3rd, so a Tuesday visit catches only one.
Walk the five spring heads between them: Tenjin (99.4°C, in the shrine grounds) and Uwanari (100.5°C, the strong smell) are the ones to see steaming.
