Futamata Radium Onsen

The defining feature of Futamata is its setting: the baths look out over a massive natural limestone travertine dome, a designated natural monument, that has been built up over centuries by the mineral-rich spring water. The mixed outdoor bath (rotenburo, 露天風呂) is free-flowing, and the water itself contains trace radium alongside calcium carbonate, magnesium, iron, and dozens of other minerals. It has a traditional toji (湯治, therapeutic bathing stay) atmosphere, simple rooms, no extras, and the spring can also be drunk.