Karuizawa Sengataki Onsen in Karuizawa, Nagano Prefecture, is a tranquil hot spring facility surrounded by the rich nature of Karuizawa. In addition to its soft, free-flowing hot spring water, the facility features a dim, low-temperature sauna that evokes the atmosphere of authentic Finland and a single-person cold plunge bath supplied with cold groundwater. Enjoying the forest air on the wooden deck during the outdoor cooling/rest period is pure bliss.