Shiro no Yu Onsen is a secret gem tucked up in the mountains of Kirishima. Not for the faint of heart, this spot is as rustic as it gets: a bath in the middle of a river has a perfect mix of both hot mineral-laden water and icy cold. Privacy is nil as there are no changing rooms; however, you'll more than likely be the only ones here, save for a surprised tanuki or squirrel.
Highly advisable to not roast the privates by inadvertently sitting down in the wrong spot (as this author did). Though the temperature of the water itself was quite nice, I did not realize that there was a scaldingly hot vent at the bottom of the pool and sat down...right on it. The rocks are also extremely slippery, so use care. Visitors to Shiro no Yu should be ready to accept all risks -- and all rewards, for it is an extremely peaceful, elemental soak -- from a trip here.
You'll need a good guide to find this place, but serious onsen lovers will have a blast -- it's a beautiful hike through some of Kirishima's prettiest mountains. When you reach the right location, strip down and poke around in the water for a spot that's the right temperature. IMPORTANT WARNING: This area is actively volcanic and there is a possibility of a poisonous gas release -- ask locals and use other precautions before deciding to risk a visit.
One final note: at certain times the water may harbor small nematode worms between a few millimeters to a centimeter long. I am not a biologist and have no idea if they pose a threat to health; however, they seemed to have no interest in me, nor did I notice any ill effects from the dip there.
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