Higashi Hot Spring

A free wild outdoor bath on the island of Iojima (between Ibusuki and Yakushima), cut into the coastal rock. The water is a strongly acidic sulfur-alum spring (pH around 1.7) with three pools at descending temperatures from the source, pick the one that suits you. No facilities whatsoever; locals keep it clean. Remote enough that getting there takes a good 40-minute walk from the ferry port.