Borexino is a detector installed at the Gran Sasso underground laboratory with the aim to study low energy neutrinos from the Sun. Until now low energy neutrinos (few MeV or less) have not been studied due to the background introduced by the natural radioactivity. The Borexino collaboration has developed new techniques to keep the natural radioactivity coming from various sources at an unprecedented low level. Borexino is able to detect also the antineutrinos produced in the mantle and the crust of the Earth by radioactive decays, following geological hypotheses which ascribe to them the production of a large part or of the total of the Earth thermic energy. In the seminar, after a description of the Borexino detector, a short report of the models concerning the radioactive decays in the Earth mantle is presented together with the main characteristics of the antineutrinos produced in these decays (geo-neutrinos). The measurements of the flux and of the energy spectrum of geo-neutrinos are discussed and the results are evaluated in terms of production of.the thermal energy.