Entangled within the earth’s five natural spheres—the atmosphere, biosphere, cryosphere, hydrosphere, and lithosphere—is a sixth: the technosphere. Identified by geologist Peter K. Haff as an emerging paradigm of the anthropocene, the technosphere includes the sites, institutions, and infrastructures of industrial production and extraction. The Sixth Sphere explores how design can participate in systems of planetary interdependence and reciprocity. Gathering the contributions of 18 practices from around the world, projects are organized around three frameworks for understanding the relationship between the built environment and the technosphere: the molecular, the machinic, and the metabolic.