Yokoi Onsen is an alkaline simple thermal spring at pH 8.66 and a generous 48.7 °C wellhead temperature. Mineralization is light (evaporation residue 691 mg/kg) and sodium-led, with a faint trace of hydrogen sulfide on the nose; the water feels notably slippery on the skin and sits squarely in Japan's beauty-water family.
385. Hirayama, Ueki Onsen, Kumamoto
Stayed overnight for 11k yen (no meals). Higaeri is also available for 500 yen, listed as 10:00 to 16:00 and 18:00 to 22:00 (closed Mondays). They stamp the 88 Onsen of Kyushu book. A simple, heartfelt old-school ryokan with a very welcoming receptionist.
Here's one of the cool stories I mentioned before. When I was checking in at the ryokan in the evening, I asked them to call a taxi for early morning to another onsen. Later I come downstairs, and the woman runs out from the reception and hands me a kind note in Russian saying they'll drive me there. A bit later she approaches me with a pencil and asks me to leave a signature on the wall where guests sign. In the morning before checkout I of course give her a matryoshka — her reaction starting with the word "matryoshka!" was very lively and positive. Through the translator she wrote that she'd only ever seen matryoshka on TV, and now in person. At the appointed time a man came separately and drove me to the onsen I wanted — no extra charge.
The indoor bath has clear water with a slight proper sulfurous onsen smell, temperature around 41. Breathing inside is comfortable.
Outdoor stone bath, clear water, temperature around 42.
Such a heartfelt place is of course recommended for a visit.
