The anomalously water-rich mantle beneath northeast China could be explained by the dehydration of subducted slabs of oceanic crust over the past billion years, according to a paper published online this week in Nature Geoscience. Takeshi Kuritani and colleagues compiled geochemical measurements of ancient lavas erupted at the Earth’s surface in northeast China, but derived from the water-rich zone in the mantle below. The geochemistry shows that water was added to this source on two separate occasions in the past one billion years. The researchers suggest that the first addition to the mantle occurred when water was squeezed out of an ancient oceanic slab as it was subducted beneath China. Later, yet more water was added during the dehydration of the subducting Pacific Ocean crust. The two mantle hydration events together can help explain the anomalously wet mantle beneath northeast China