The deep Earth holds about the same amount of water as our oceans. That’s the conclusion from experiments on rocks typical of those in the mantle transition zone, a global buffer layer 410 to 660 kilometres beneath us that separates the upper from the lower mantle. “If our estimation is correct, it means there’s a large amount of water in the deep Earth,” says Fei Hongzhan at the University of Bayreuth in Germany. “The total