Yudomari Onsen

A bicarbonate-chloride spring on remote Akusekijima in the Tokara Islands, with both a small indoor tub and an open-air section. The water runs naturally hot and slightly sulfurous, leaving skin noticeably soft. Getting here is already an adventure -- island-hopping to one of Japan's most isolated inhabited islands -- and the spring itself is charmingly bare-bones, unmanned, and run on the honour system.