Architecture, Building and Structure Design Ondsauna Sauna by Taro SHIMOKAWA Trees grow endlessly upward, a concept reflected in the vertical exterior walls clustered around the sauna room. A spiraling path leads upward to a rooftop relaxation space, mirroring the building's form. The sauna entrance is at GL+250, the cold plunge deck at GL+1000, and the rooftop at GL+3250, so the transition from sauna to cold plunge to open air follows a vertical flow. Viewing the forest at treetop height allows one to share in its growth rhythm.