Located in Hongu-cho, Tanabe City, Wakayama Prefecture, Tsuboyu at Yunomine Onsen is one of Japan's oldest hot springs, said to have been discovered approximately 1,800 years ago by Oato no Sukune, the provincial governor of Kumano. Registered as a UNESCO World Heritage Site in 2004 as part of the Sacred Sites and Pilgrimage Routes in the Kii Mountain Range, it is the only World Heritage Site in the world where visitors can actually bathe. The spring water, which rises in a valley along the Yomura River, is a sulfur-containing sodium bicarbonate and chloride spring with a source temperature of approximately 90-92.5 degrees Celsius and pH 7.8 (weakly alkaline). Tsuboyu's most remarkable feature is its mystical color-changing phenomenon, said to occur seven times daily, shifting between milky white, bluish-white, and transparent due to changes in mineral composition and temperature. The tiny natural rock bath, which fits only 2-3 adults, operates on a 30-minute rotation system. The spring is also famous for the legend of Oguri Hangan, a Muromachi-period warrior who was poisoned and brought here by his wife Princess Terute, where he underwent 49 days of therapeutic bathing and was miraculously restored to health, earning it the name Spring of Resurrection. Since ancient times, Kumano pilgrims have used this spring for yugori purification rituals. The village features Yuzutsu, a spot where 90-degree-plus source water wells up, which visitors can use to boil eggs and vegetables. The spring flows freely from the source without dilution, boasting an abundant output of 885 liters per minute.