From Archean magma oceans to present-day production and migration of silicate melts beneath mid-ocean ridges, in subduction zones, and potentially in various settings of the deeper mantle, magma plays a central role in the physical andchemical evolution of the Earth. Low seismic velocity regions in the mantle and crust are commonly attributed to the presence of silicate melts. Determining melt volume and geometric distribution in the Earth’s interior is fundamental to understanding planetary dynamics. Using recent laboratory experiments, I will show that silicate melts do not behave at high pressures as commonly predicted when using standard equations of state. I will present a new model for how silicate melts affect seismic wave speed in the upper mantle and crust, and using this model, reinterpret the role of melt at the lithosphere-asthenosphere boundary.