Subsurface environments beneath the continents and the oceans represent the most expansive habitat on Earth, yet that which we know the least about. Our laboratory studies the diversity, distribution, and activities of microorganisms in the deep subsurface biosphere using molecular biological approaches coupled with geochemical analyses. Our focus is on high pH (10-12) microbial ecosystems created by the serpentinization of ultramafic rocks from the deep Earth. We also study high temperature microbial ecosystems associated with volcanically-driven hydrothermal venting in the oceans. Investigation of these ecosystems will provide insight into the contributions of subsurface microorganisms to the global carbon budget, and may have applications ranging from biotechnology and alternative energy strategies.