Sea water does filter down through cracks in the ocean floor but as it does, it encounters hotter and hotter rocks. Some of the water is then expelled upward to the sea floor as superheated fluid, producing "black smokers" and other mineral-laiden jets of hot water. The rest reacts with the minerals in the rocks, changing them from anhydrous (water-absent) minerals to hydrous (water-containing) minerals. In this way, the rocks of the ocean floor become partially hydrated. When ocean floor is subducted into the mantle at a subduction zone (also called a "destructive plate boundary") the hydrous minerals become too hot to hold onto their water and they release water into the overlying mantle and crust, and once again become anyhdrous.