Santogoya Onsen sits at about 1,460 m on the western flank of Mt. Asahi in the Nasu mountain range, Nasushiobara City, Tochigi Prefecture. Discovered in 1142, it’s said to be the oldest spring in Nasu and was one of the “Seven Springs of Nasu” during the Edo period.

The main inn, Kuroya, is a historic wooden building dating from 1869 (Meiji 2) and comprises 21 guest rooms, a terrace, and a large communal hall. Inside you’ll find proper changing rooms and flush toilets. As a true mountain hut, it provides basic lodging facilities, but no road reaches it—guests arrive on foot, and after 9 p.m. the only light comes from oil lamps, preserving the atmosphere of an old-style healing retreat.