Tsukahara Onsen is a different kind of Beppu bath: rougher, more remote, and more mineral-heavy than the polished city baths. Set in the hills toward Yufuin, it is known for strongly acidic water and a volcanic setting that makes the soak feel close to the source rather than packaged for easy sightseeing.
This is a good choice for travellers who have already tried Takegawara or the Kannawa baths and want something more elemental. Facilities are simple, the water can feel sharp on sensitive skin, and getting there takes more effort than staying in town. That effort is exactly why it stands out.