Unzen Mikado Hotel Honkan

The indoor bath here is fed by a hot spring that rises on the property itself, with a wide washing area and a sauna alongside it. Outside, an open-air bath cut from natural stone looks across the Ariake Sea toward Kumamoto's Amakusa islands; the men's and women's sides swap between morning and night, so staying over means trying both, including the sunrise side. The water itself is a plain, non-mineral spring that feels light and smooth rather than strong. This is a large hotel on the sea-facing side of the Shimabara Peninsula, not up in the mountain onsen town the Unzen name might suggest, known as much for its crab and wagyu buffet dinners as for its baths.