立岩海底温泉 is a hot spring town in Yamagata, Japan. The water leaves the ground at 50°C, hot. Officially the water is classed as chloride spring and sulfate spring.
Salty water. The salt leaves a thin film on the skin, so you stay warm well after leaving the bath.
Clear water with a faintly bitter taste. Known for holding heat.
Figures from the town's Japanese Wikipedia article.
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