Hot vents in the deep sea and geysers on land document the penetration of water into the hot interior of the Earth. This happens primarily in regions where the crust breaks up and magma chambers are close to the surface, e.g. in the area of mid-ocean ridges. But how deep does the water penetrate and cool the upper part of the hot mantle? So far, it has been assumed that this process only reaches depths of a few kilometres. A new analytical method developed at GEOMAR Helmholtz Centre for Ocean Research Kiel now shows that water penetrates much deeper into the Earth than previously thought.