Aya researched natural hot springs before we left. This one wasn't exactly on the list, but we could smell it while we were looking for a different one. Though it was on private land, there were no signs warning against trespass and the road had no gate.
Here's the Google Maps link.
The large pool was lukewarm, but the white mineral deposits on the far side suggested something better.
Lovely hot water bubbling out of connected pools, from about a meter to two meters deep. We quickly became accustomed to the light sulfur smell and enjoyed a long, glorious soak, listening to the gurgling sounds of the springs echoing under the hollow rock formations.
We saw pools like this in Yellowstone NP many years ago but of course they were off-limits. It was amazing to be able to gently slip into one and enjoy the hot water.
Lightning flashed in the sky around us and the sun set behind storm clouds while we set up our tent without a drop of rain.