The cluster of concrete spires and hulking domes, two miles off Interstate 17 at Cordes Junction, welcomes 50,000 visitors a year with a gift shop, communal cafeteria, and bakery. Residents, who come here to learn while helping with the slow but continual construction, lead hour-long tours ($8 donation) that illuminate Soleri’s vision of arcology—architecture married to ecology. Rambling through the earth-friendly apartments and workspaces, visitors discover how high-density living might help curb society’s hyperconsumption.