At Kawayu, where 73°C spring water bubbles up through the bed of the Ōtō River, a giant outdoor bath is created every December to February by damming the river. Roughly 50 by 15 metres — big enough for 'a thousand people', as the name says — it's tempered to perfection by the river's cold flow. Free (donation box), 6:30–22:00, swimwear required, open to everyone. Bathing is suspended in bad weather or high water and opening dates shift from year to year. In summer you can dig your own pool in the gravel. Combine with Yunomine and Watarase for the three Hongu baths.

✔ Verified operating: 2026-09 熊野本宮観光協会・駅探（2025/12/1–2026/2/28・6:30–22:00・無料・水着）※次季の開設日は観光協会で要確認

Prices and hours change. Check official sources before visiting.

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