Hot springs are a small miracle of nature, fed by rainwater that seeps into the rock that is heated by the earth's interior before bubbling up to the surface, a process that takes years, even decades. More than 13,000 onsen inns and baths dot the country. There are strict rules, displayed in numerous languages on posters plastered on onsen walls. No bathing suits. No soapy bodies allowed. And an additional Covid-era requirement, "mokuyoku," or silent bathing — no chatter in the baths. Geothermal power plants, on the other hand, draw on wells drilled deeper in the earth's crust, pumping up steam and hot water to power giant turbines that generate electricity. Developers say that because plants draw from sources deep beneath onsen springs, there is little possibility one will affect the other. Still, the interconnection between hot springs and deeper geothermal heat remains something of a mystery. When hot spring flows change, it's often difficult to pin down a cause.