Ubayu Onsen Masugataya

Set at 1,300 meters in a deep Yamagata gorge, Ubayu is ringed on three sides by sheer rock cliffs, the scenery alone would justify the trip. The mixed open-air bath (Yamamba no Yu, 山姥の湯) sits right against the cliff face, fed by a 51°C acidic sulfur spring that runs unheated and undiluted. The inn has been in the same family for 17 generations, with a history going back to 1533 when, according to legend, a mountain crone asked the first innkeeper to tend the baths.