The archaeological record shows that large pre-Inca agricultural systems supported settlements for centuries around the ravines and oases of northern Chile's hyperarid Atacama Desert. This raises questions about how such productivity was achieved and sustained, and its social implications. Using isotopic data of well-preserved ancient plant remains from Atacama sites, we show a dramatic increase in crop nitrogen isotope values (delta N-15) from around ad 1000. Maize was most affected, with delta N-15 values as high as +30 parts per thousand, and human bone collagen following a similar trend; moreover, their carbon isotope values (delta C-13) indicate a considerable increase in the consumption of maize at the same time. We attribute the shift to extremely high delta N-15 values-the highest in the world for archaeological plants-to the use of seabird guano to fertilize crops. Guano-'white gold' as it came to be called-thus sustained agricultural intensification, supporting a substantial population in an otherwise extreme environment. The source of pre-Inca agriculture in the Atacama Desert of Chile has been the subject of multiple theories, but this Article uses preserved maize remains to deduce that coastal guano deposits were utilized in an impressive display of social and ecological sophistication.