Kawayu Onsen is an exceptionally rare hot spring located in Hongu-cho, Tanabe City, Wakayama Prefecture, situated along the Oto River, a tributary of the Kumano River. Just a 10-minute drive from Kumano Hongu Taisha, part of the UNESCO World Heritage site "Sacred Sites and Pilgrimage Routes in the Kii Mountain Range," this hot spring has long been cherished as a place of healing for pilgrims traveling the Kumano Kodo ancient trails.

The most distinctive feature of this hot spring is that thermal water emerges wherever you dig in the riverbed. Hot spring water at 73°C continuously bubbles up from the river bottom, mixing naturally with the cool stream water of the Oto River to create the perfect bathing temperature. During summer, visitors can bring their own shovels to dig the riverbed and create their own personalized open-air bath, making it a uniquely interactive hot spring experience.

In winter (annually from December to February), when the river's water level drops, the Oto River is dammed to create "Sennin-buro" (Hermit Bath), one of Japan's largest open-air baths measuring approximately 24 meters wide, 12 meters deep, and 60 centimeters in depth. This spectacular bath has been constructed annually since 1985 by the local Kumano Hongu Tourism Association and has become the winter hallmark of Kawayu Onsen. Named "Sennin-buro" because it can accommodate a thousand people (sennin), this massive outdoor bath offers an unparalleled sense of openness while bathing under blue skies or starlit nights. Every Saturday night features "Yukemuri Toro" (Steam Lantern) events, where lanterns illuminate the bath creating a mystical atmosphere.

The hot spring has ancient origins, with records indicating use since the early Edo period. The discovery of the source and establishment of a hot spring inn (now Fujiya) during the Edo period marks the beginning of Kawayu Onsen. In 1957, it was designated as a National Health Resort along with Yunomine Onsen and Wataze Onsen as part of the Kumano Hongu Hot Spring Village.

The water quality is sodium-bicarbonate/chloride hot spring, known for its therapeutic effects on neuralgia, muscle pain, joint pain, diabetes, and sensitivity to cold. When consumed, it is said to benefit chronic digestive disorders, chronic constipation, gout, and liver disease. The hot spring town also features "Kawayu Onsen Public Bath," a traditional communal bathhouse where day-trip bathing is readily available.

Kawayu Onsen is highly regarded among hot spring enthusiasts as a wild and natural hot spring where one can experience nature's gifts firsthand. Combined with visits to the World Heritage Kumano Kodo pilgrimage routes, it offers a precious opportunity to experience both Japan's ancient pilgrimage culture and hot spring traditions.