Futamata Onsen Daimaru Asunaroso is a secluded single-inn hot spring ryokan nestled along the Futamata River gorge at approximately 800 meters elevation in Tenei Village, Iwase District, Fukushima Prefecture. Discovered in 969 CE during the Heian period, this hot spring boasts over 1,050 years of history and is celebrated as the founding inn of the Japan Association of Secluded Hot Spring Inns (Nihon Hitou wo Mamoru Kai). The current proprietor, Satoh Koui, representing the 50th generation, serves as the association's honorary chairman. The inn's most remarkable feature is the self-spouting pothole rock bath (Jifunsen Ouketsu Iwaburo), where hot spring water naturally wells up through ancient potholes formed in the bedrock of a prehistoric riverbed - an extremely rare phenomenon nationwide. Guests can feel the source water bubbling up directly beneath their feet. The water is calcium sulfate spring (formerly classified as pure gypsum spring) with pH 8.6-8.8, source temperatures of 50.8-53.6 degrees Celsius, fed by six private sources all delivered as 100% natural free-flowing water. The colorless, odorless, and tasteless water is rich in calcium ions with notable sedative properties, traditionally known as a wound-healing spring. The riverside outdoor baths offer views of the Futamata River surrounded by primeval beech forests, and the women-only fertility bath (Kodakara Rotenburo) is also popular. The inn has been operating since the Edo period under the name Daimaruya for 50 generations, and was renamed Asunaroso by Prince Takamatsu after reconstruction following record snowfall in 1964. Designated as a National Health Resort Hot Spring.