An in-depth look at a significant aspect of the Earth system. The "critical zone" -- defined as the outer layer reaching from the treetops into the weathered bedrock where rock, water, air and life interact -- is crucial to life and habitats, food production, water quality, and regulating climate. Topics will vary depending on instructor expertise, who may focus on biosphere-atmosphere interactions, or microbiologically driven cycling of energy and matter in the subsurface. Emphasis will be placed on approaches to quantitatively model interactions and reading of primary scientific literature.