Nature produces hydrogen through "serpentinization." When water meets the ubiquitous mineral olivine under pressure, the rock absorbs mostly oxygen (O) atoms from H2O, transforming olivine into another mineral, serpentine -- characterized by a scaly, green-brown surface appearance like snakeskin. The complex network of fracturing and created by serpentinization also creates habitat for subsurface microbial communities. Image from Gros Morne National Park, Newfoundland, Canada. Credit: Matt Schrenk, Michigan State University