Yunomine Onsen Tsuboyu
The only hot spring inscribed as a World Heritage Site that you can actually bathe in, listed together with the Kumano Kodo pilgrimage routes. A tiny hut over the stream in Yunomine — said to be Japan's oldest onsen at 1,800 years — shelters a tub hollowed from natural rock that fits two adults. The sulfurous water is said to change colour seven times a day and served pilgrims for ritual purification. Private 30-minute slots, ¥800, sign up at the public bathhouse (6:00–21:00). Waits of several hours are common on holidays; boil onsen eggs in the hot-spring trough while you wait.