Gold 's theory is that flow of food is due to out-gassing of primordial methane from the Earth 's mantle ; more conventional explanations of the food supply of deep microbes ( away from sedimentary carbon compounds ) is that the organisms subsist on hydrogen released by an interaction between water and ( reduced ) iron compounds in rocks Gold 's theory is that flow of food is due to out-gassing of primordial methane from the Earth 's mantle ; more conventional explanations of the food supply of deep microbes ( away from sedimentary carbon compounds ) is that the organisms subsist on hydrogen released by an interaction between water and ( reduced ) iron compounds in rocks .