Kannojigoku Hotel

Kannojigoku is unusual in the best way: it runs a cold hydrogen-sulfide spring in summer (July to September) alongside the hot baths, and soaking in near-icy sulfurous water is genuinely bracing. The lodge has a strong old-Japan feel, crackling firewood, dim corridors, home cooking around the hearth, and the remoteness (three-plus hours from Oita airport by bus) keeps the crowds away.