Let’s be clear about what we mean by ‘geothermal’ heat

The article Hidden Depths (23 November, page 58) repeats an error which we should not be making about the term “geothermal” heat. Geothermal heat is heat of volcanic origin and nothing to do with ground water or earth-source heat. Iceland has geothermal heat - Milan has ground or water-source heat.

Richard Saxon, London