Nukabira Onsen Sankoso

Deep in the mountains of Hokkaido's Nukabira hot spring village, this small inn has a cave bath fed by a sodium chloride and bicarbonate spring, not large, poured in concrete, but genuinely cave-like in atmosphere. Rooms come with traditional hearth fireplaces (irori, 囲炉裏), and the kitchen leans into local mountain forage: wild vegetables, river fish, the real deal.