An onsen resort about 15 km south of the central area of Aizuwakamatsu City, with ryokan lined up along the gorge of the O River. Surrounded by abundant nature, it has a history of being called a "phantom hot spring." In particular, Aizu Ashinomaki Onsen Ookawaso drew worldwide attention through social media for its floating stage, which closely resembles the "Infinity Castle" from the popular anime Demon Slayer: Kimetsu no Yaiba, and saw a sharp increase in inbound tourists. Offering both a spectacular open-air bath with views of the beautiful gorge and an experience that feels like a pilgrimage to a sacred site for Japanese pop culture, especially anime, it is attracting attention as a new kind of inbound destination.