Kuronagi Onsen, located in Unazuki-machi, Kurobe City, Toyama Prefecture, is the oldest hot spring in the Kurobe Gorge and serves as the source for Unazuki Onsen. Discovered in 1645, it was once a forbidden hidden hot spring under the Kaga Domain until its opening was permitted in 1868. This secluded single-inn retreat can only be reached by the Kurobe Gorge Railway trolley train. With an abundant flow of 2,000 liters per minute, it supplies all the hot spring water to the Unazuki Onsen resort below. The water quality is simple thermal spring (low-tension, weakly alkaline, high-temperature spring) with a source temperature of 98.3 degrees Celsius. The colorless, transparent, and soft water makes skin silky smooth and has been renowned as a miraculous healing spring since ancient times, said to eliminate the need for doctors or medicine. The mixed outdoor bath Izumi spans 28 tatami mats, the largest along the Unazuki line, offering spectacular gorge views from the riverbank. Open seasonally from early May to late November.