Our planet may be blue from the inside out. Earth’s huge store of water might have originated via chemical reactions in the mantle, rather than arriving from space through collisions with ice-rich comets. This new water may be under such pressure that it can trigger earthquakes hundreds of kilometres below Earth’s surface – tremors whose origins have so far remained unexplained. That’s the upshot of a computer simulation of reactions in Earth’s upper mantle between liquid hydrogen and quartz, the most common and stable form of silica in this part of the planet.