At the start of this book, we were reminded that the Earth's thermal evolution is an old question that was first addressed by Kelvin. Our understanding of this problem has been turned upside down several times since the time of Kelvin, but the questions that he raised have not all been fully answered. The length of this book, which does not even cover everything we know, is a clear demonstration of the enormous progress that we have made since 1862. Still, many questions remain, and understanding the Earth's thermal evolution is still a major challenge.

Thanks to decades of careful measurements, we know well the total energy loss of the Earth, and we know that the largest fraction of this energy is brought to the surface by convection in the Earth's mantle. Mantle convection is driven by secular cooling of the Earth, radiogenic heat production in mantle rocks, and heating from the core. The balance between these three sources of energy is very poorly constrained and it is likely to remain one of the most significant stumbling blocks in studies of Earth's evolution through geological time. The development of neutrino observatories opens new and exciting perspectives for the geosciences. It will require many years of observations with land detectors, and the deployment of observatories on the sea floor to narrow down our estimates of the heat production of the mantle but there is real hope that, in the end, we shall have stronger constraints on one of the sources that enters the budget.