This is onsen at its most elemental: natural hot pools carved into the rocks beside the ocean, accessible only at or near low tide. When the sea retreats, the baths emerge steaming from the volcanic rock, and you'll soak with waves crashing just metres away. It's a mixed-bathing spot where swimsuits are not allowed — women traditionally wrap in a thin towel — adding a layer of vulnerability that somehow intensifies the experience. Check the tide tables, walk down from the bus stop, and surrender to one of the most unforgettable baths in Japan.