Aso Uchinomaki Onsen

Aso Uchinomaki Onsen sits within the vast Aso caldera — the largest active caldera in the world at 25 kilometers across, where the living volcanic landscape of smoking Nakadake crater, grassland prairies grazed by wild horses, and geothermal vents across the caldera floor create the most dramatically active volcanic setting of any onsen in Japan. Calcic spring water from the caldera's geothermal activity fills bathhouses and ryokan baths in the Uchinomaki district, where farming communities have soaked in caldera volcanic water for centuries beneath the smoking summit of Nakadake — still regularly emitting sulfurous gases and occasional ash clouds visible from the pools. The extraordinary experience of soaking in caldera spring water while looking directly at an active volcano in the world's largest caldera makes Aso Uchinomaki one of Japan's most viscerally alive and geologically spectacular onsen destinations.