Hydrogen-producing Earth rocks may hold a key to extraterrestrial life forms. Scientists have long known that hydrogen gas is set free from many common rocks when water infiltrates fresh cracks within them. Primitive bacteria that comprise a subterranean biomass possibly outweighing all living things at Earth’s surface thrive on this hydrogen, using it as an energy source. A recent study describes a different hydrogen-producing reaction within the minerals that make up the bulk of certain rocks that were once hot or molten. This latter process could indicate a novel support system for microbes on other planets. Friedemann Freund of NASA’s Ames Research Center suspects that similar hydrogen-consuming microbes may exist deep in the rocks on Mars and other planets that contain water today or were once wet.