Discover Shiriyaki Onsen – Japan’s Wildest Natural River Hot Spring! Hidden deep in the mountains of Gunma Prefecture, Shiriyaki Onsen (尻焼温泉) offers one of the most unique onsen experiences in Japan: the entire river becomes your bathtub! The star here is “Kawa no Yu” (River Bath) — a massive, free, open-air mixed onsen created by damming part of the Nagasasa River. Hot spring water gushes directly from the riverbed, heating the rocks and pools naturally. Sit on the warm stones, let the current flow around you, and feel completely merged with nature....