Melting in the Earth’s mantle rules the deep volatile cycles because it produces liquids that concentrate and redistribute volatile species. Such redistributions trigger volcanic degassing, magma emplacement in the crust and hydrothermal circulation, and other sorts of chemical redistribution within the mantle (metasomatism). Melting also affects mantle viscosities and therefore impacts on global geodynamics. So far, experimental petrology has been the main approach to construct a picture of the mantle structure and identify regions of partial melting.Magnetotelluric (MT) surveys reveal the electrical properties of the deep Earth and show highly conductive regions within the mantle, most likely related to volatiles and melts. However, melting zones disclosed by electrical conductivity do not always corroborate usual pictures deduced from experimental petrology. In 2008, I proposed that small amount of melts, very rich in volatiles species and with unusual physical properties, could reconcile petrological and geophysical observations. The broadening of this idea is however limited by (i) the incomplete knowledge of both petrological and electrical properties of those melts and (ii) the lack of petrologically based models to fit MT data. ELECTROLITH will fill this gap by treating the following points:- How volatiles in the H-C-S-Cl-F system trigger the beginning of melting and how it affects mantle conductivity?- What are the atomic structures and the physical properties of such volatile-rich melts?- How can such melts migrate in the mantle and what are the relationships with deformation?- What are the scaling procedures to integrate lab-scale observations into a petrological scheme that could decipher MT data in terms of melt percolation models, strain distributions and chemical redistributions in the mantleELECTROLITH milestone is therefore a reconciled perspective of geophysics and petrology that will profoundly enrich our vision of the mantle geodynamics