The Earth as a source of heat
Geothermy

All the obvious manifestations of our planet's thermal energy (volcanoes, fumaroles, hot springs, etc.) were known to our distant ancestors, and the earliest uses of geothermal energy are lost in the mists of time: hot springs for their curative virtues, cooking and drying food thanks to fumaroles, mining thermal springs to recover boric acid and extracting sulfur and salt.

However, all these applications remained on an extremely modest scale, and it wasn't until the early 20th century...