Arid lands, which cover some 40 percent of the Earth’s terrestrial surface, are too dry to sustain much in the way of vegetation. But far from being barren, they are home to diverse communities of microorganisms—including fungi, bacteria, and archaea—that dwell together within the uppermost millimeters of soil. These biological soil crusts, or biocrusts, can exist for extended periods in a desiccated, dormant state. When it does rain, the microbes become metabolically active, setting in motion a cascade of activity that dramatically alters both the community structure and the soil chemistry.