Located in Fukawa-Yumoto, Nagato City, Yamaguchi Prefecture, Onto is the central public bathhouse of Nagato Yumoto Onsen, the oldest hot spring in Yamaguchi. According to legend, the spring was discovered in 1427 (Oei 34) by Joan Shuzen, the third chief priest of Taineiji Temple of the Soto Zen sect, who received a divine revelation from the deity Sumiyoshi Daimyojin. With over 600 years of history, it is celebrated as a sacred hot spring bestowed by the gods, and the source rights still belong to Taineiji Temple to this day. Reopened in March 2020 after a major renovation, the building is uniquely constructed directly above the spring source itself, allowing bathers to actually see the hot water gushing out from the bedrock, a feature extremely rare nationwide. The water is an alkaline simple mineral spring with a pH of 9.62, naturally emerging at the bathers' feet at around 39 degrees Celsius. It is supplied as pure free-flowing source water (overflow style) without any recirculation. The colorless, slightly bluish water carries a subtle sulfur fragrance and offers an exceptionally smooth, silky touch. The standing-bath style tub is approximately one meter deep, allowing visitors to fully immerse themselves in this divinely-bestowed water.