Tomarikawa Onsen Tomarikawa Kajika no Yu

A genuine wild spring on the far bank of the Tomarikawa River in Shimamaki, where iron-red water seeps up from the riverbed and has built up a striking red mineral dome over the years. Reaching it involves crossing the river and a bit of scrambling, and the bathtub itself, buried under mineral deposits and fallen leaves, may or may not be usable depending on recent water flow and conditions. The source is real and strong; the infrastructure is near zero. Worth it for the adventure and the scenery, but come prepared to be flexible.