The symbol of Nagayu, said to have Japan's most carbonated water. Sink into the 32°C open-air pool and silver bubbles coat your whole body. The charred-cedar and plaster building by architect Terunobu Fujimori is a sight in itself.
The flagship bathhouse of Nagayu Onsen in Ōita, which bills itself as Japan's best carbonated spring. Its European look comes from a sister-city tie with Bad Krozingen in Germany, and inside the water flows straight from the source with no heating or dilution. The rust-brown sodium-bicarbonate water is rich in iron and CO₂, and fine bubbles cling to your skin in the tub. Gender-separated indoor and open-air baths on the 1st and 3rd floors swap daily. Open 06:00–21:00 (last entry 20:00) — great for an early soak; closed the 3rd Wednesday of each month. Adults from outside the city pay ¥700; a private family bath is ¥2,400 for 50 minutes. Cards and QR payment accepted. 25 minutes by car from JR Bungo-Taketa Station.
Three minutes on foot from Gozen-yu, this is the gender-separated communal bath locals use every day. You pay ¥300 at a ticket machine and walk in — no attendant, and the bathroom holds just one indoor tub. Nagayu's bicarbonate water flows freely and has stained the spout area brown with mineral deposits. Open 06:00–22:00 (last entry 21:30), closed the 4th Wednesday. No soap or shampoo, so bring your own. If you are walking around Nagayu, add this ¥300 locals' bath to Gozen-yu and the free riverside Gani-yu.