Of all the habitable parts of our planet, one ecosystem still remains largely unexplored and unknown to science: The igneous ocean crust. This rocky realm of hard volcanic lava exists beneath ocean sediments that lie at the bottom of much of the world's oceans. While scientists have estimated that microbes living in deep ocean sediments may represent as much as one-third of Earth's total biomass, the habitable portion of the rocky ocean crust may be 10 times as great....