While the weather in Iceland is often cold, wet, and windy, a nearly endless supply of heat bubbles away below the surface. In fact, almost every building in the country is heated by geothermal energy in a process with virtually no carbon emissions. So how exactly does this renewable energy work? Jean-Baptiste P. Koehl explores the two primary models for harnessing the planet's natural heat. Lesson by Jean-Baptiste P. Koehl, directed by Charlotte Arene.