In the deep, chemically reducing parts of Earth's mantle(1), hydrous fluids contain significant amounts of molecular hydrogen (H-2). Thermodynamic models of fluids in Earth's mantle so far have always assumed that molecular hydrogen and water are completely miscible. Here we show experimental evidence that water and hydrogen can coexist as two separate, immiscible phases. Immiscibility between water and hydrogen may be the cause of the formation of enigmatic, ultra-reducing domains in the mantle that contain moissanite (SiC) and other phases indicative of extremely reducing conditions(2,3). Moreover, the immiscibility between water and hydrogen may provide a mechanism for the rapid oxidation of Earth's upper mantle immediately following core formation(4).