Natural onsen near Yokosuka with high-ceilings and pleasant open-air baths. Perhaps a little out of the way, but it has ample parking for those visiting by car.
Noborigumo (のぼり雲) is an inviting local onsen with open-air baths, sauna, and a pleasing facade. The water is pumped from 800 meters below ground level and flows out at a temperature of 32.5°C. From here it is re-heated to about 42-43°C for the baths—perhaps a little hotter than most onsen. The water itself is mineral rich and clear, although certain elements in the water colour upon contact with air.
The onsen building itself is conical with natural light pouring in from the top, creating a feeling of spaciousness. There is just one indoor bath—a rectangular tub where the water is hottest. Outside there are two baths, one with natural stone surrounding the tub, as well as a walk-in bath set at a much cooler temperature where visitors can stride through the water. There is also a standard sauna, mizu-buro, and low-temperature salt sauna.