Splitting water into hydrogen and oxygen is a major theme in the development of clean, abundant energy source. A new study led by an international research group revealed that when water meets the iron core of the Earth, the extremely high pressures and temperatures existing at the core-mantle boundary can naturally cause water to split into hydrogen and a super oxidized iron dioxide. Both the released hydrogen and the retained oxygen in the dioxide have many far-reaching implications and consequences, including the behaviors of the core-mantle boundary as a huge hydrogen generator, the separation of the deep Earth’s water and hydrogen cycles, and the accumulation of oxygen-rich patches.