In a pristine corner of the Amazon, Brazilian and British scientists have spent more than two decades simulating drought to understand how the rainforest might respond to a drier future. The Esecaflor experiment, launched in 2000, used plastic panels to block about half the rainfall over a one-hectare plot, mimicking long-term water stress. The result is a dramatic decline in biomass and the death of many large trees, with roughly 40% of the vegetation lost. As droughts become more frequent and intense, understanding how soil, roots and trees respond is critical to predicting the future of the Amazon.