Our body temperature is held at a fairly steady 37.5°C, and the assumption has always been that most of our physiological processes take place at this temperature. The heat needed to maintain this temperature in the face of a colder environment is generated by tiny subcellular structures called mitochondria. But a new study publishing January 25 in the open access journal PLOS Biology by INSERM and CNRS researchers at Hôpital Robert Debré in Paris led by Dr Pierre Rustin (and their international collaborators from Finland, South Korea, Lebanon and Germany) presents surprising evidence that mitochondria can run more than 10°C hotter than the body’s bulk temperature, and indeed are optimized to do so. Because of the extraordinary nature of these claims, PLOS Biology has commissioned a cautionary accompanying article by Professor Nick Lane from University College, London, an expert on evolutionary bioenergetics.