四季の湯温泉 is a hot spring town in Saitama, Japan. The water leaves the ground at 27.3°C, cool enough that the baths here are usually reheated. It measures pH 7.6 — mildly alkaline, the slippery kind. Officially the water is classed as chloride spring. 1 inns take visitors here.
Salty water. The salt leaves a thin film on the skin, so you stay warm well after leaving the bath.
Figures from the town's Japanese Wikipedia article.
Planning a first soak? Read how an onsen stay works, and what to do if you have tattoos.