Across most of the U.S., residential geothermal heating systems depend on electric heat pumps to boost low-grade heat collected from soil or groundwater to temperatures high enough to be useful. But the lucky inhabitants of a few geologically privileged areas can heat their homes by tapping directly into naturally occurring hot groundwater.

One such place is Klamath Falls, Ore. Most of the city's downtown core - including its sidewalks - has been heated by an elaborate municipal geothermal plant since the early 1990s. Area homes and ranches, however, have relied on simpler systems at least since the 1930s.

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