Scientists have discovered a hidden layer of Earth, which sits 100 miles below the surface and covers at least 44 percent of the planet. This previously unknown region of molten rock is part of the asthenosphere, located under tectonic plates in the upper mantle, which forms a soft boundary that allows the solid rock slabs to move. While the discovery is significant, it shatters long-held theories that molten rocks influence the asthenosphere's viscosity. Junlin Hua, with the University of Texas, Austin, said in a statement: 'When we think about something melting, we intuitively think that the melt must play a big role in the material's viscosity. 'But what we found is that even where the melt fraction is quite high, its effect on mantle flow is very minor.'