Microorganisms are the Earth's firstborn. They were teeming on the land and in the seas for billions of years before the evolution of plants and animals, and their diversity is staggering. Every pinch of soil contains billions of microbes whose activities regulate terrestrial nutrient cycles and influence the evolution of our biosphere. Despite their importance many aspects of soil microbial communities are still a mystery and soil remains a fertile habitat for discovery. We are using genomic tools to examine the microbial communities that live in soils, to explore their diversity, and to delineate the mechanisms by which they influence environmental processes in terrestrial ecosystems.