Kajiyama Yukawa Onsen Kajiyama Motoyu

A cave hot spring deep in the mountains of Niigata, accessible only by reservation for one group per day. Getting there means a roughly hour-long trek from the parking area with rope-assisted climbs in places -- genuinely demanding. The cave itself pools water that's on the cooler side, so you can soak as long as you like. One of those places that earns the label "hito no naka no hito" (秘湯中の秘湯, a hidden spring among hidden springs).