Unzen Shinyu Onsen-kan

An old-school sento for 200 yen, absolutely magnificent in its jankiness: no showers — instead, each washing station has two faucets (hot with onsen water and cold with regular water), plus yellow basins for mixing. No toiletries provided, towels are bring-your-own only. Among the regulars there are tattooed old guys. One indoor bath, fairly deep, the water is slightly cloudy with a strong hydrogen sulfide smell, scorching hot at 46.2°C (though the temperature varies since it's kakenagashi).