Kawa-yu Onsen Wakayama River

Kawa-yu Onsen is one of Japan's most extraordinary natural thermal phenomena, where the Ōtōgawa river itself becomes a giant geothermal bath each winter as bicarbonate-hot spring water at 70°C bubbles up through the riverbed — visitors dig personal pools in the river gravel where the volcanic and river water mix to ideal bathing temperatures in the open air along the entire river corridor. The communal winter riverside Sennin Buro bath, created each November along hundreds of meters of the Ōtōgawa riverbank, is one of Japan's most primally natural and democratically communal bathing experiences, continuing the ancient purification traditions of the sacred Kumano Kodo pilgrimage region. Set within the UNESCO Sacred Sites of the Kii Mountain Range and a short walk from the great Kumano Hongu Taisha shrine, Kawa-yu's riverbed onsen is among Japan's most spiritually resonant and geologically extraordinary thermal experiences.