Records of the onsen date back as far as 110 AD. A wounded warrior is said to have drawn an arrow out of his body and cleaned the wound at a spring only to find that the injury healed miraculously quickly and well. It is known locally as "Springs of Beauty."
The hot mineral water has a high acid and sulfur content, giving it the characteristic smell of rotten eggs from the sulphur. Other minerals include iron, aluminum, sulfate and chloride. The water temperature at the open airpublic bathhouse is 113 °F/45 °C.
Snow monster phenomenon
The Zao Mountain is known for its conifer trees which become encrusted and solidified by January due to the hard rime phenomenon, which is caused by heavy snow and winds of freezing temperatures. These formations are known as "snow monsters" or juhyo in Japanese. They are caused by the frigid storm winds of the Siberian jet stream.