Misasa Onsen — Radium Hot Springs in River Gorge

Misasa Onsen, 20 minutes by bus from Kurayoshi, is one of Japan's most unique hot spring towns, famous for its radon-rich (radium) waters believed to have therapeutic properties. The town sits in a narrow river gorge along the Mitoku River, with traditional ryokan lining both banks and a public footbath running along the riverside path. The name 'Misasa' means 'three mornings,' referring to the belief that bathing in the hot spring for three consecutive days cures ailments — a claim that has drawn visitors for 900+ years.

The signature feature is Kawarayu Onsen (河原風呂), a free open-air bath built directly into the riverbed. The bath is mixed-gender and requires swimwear (unlike traditional onsen), creating a unique public bathing culture where locals and tourists soak together while watching the river flow past. The water contains radon concentrations among the highest in the world, and while the health claims are debated, the riverside setting and communal atmosphere are undeniably restorative.