Across five thousand kilometres of open ocean, a desert and a rainforest maintain a quiet, ancient partnership: dust lifted from a dried Saharan lakebed carries the precise mineral the Amazon needs to replace what its rivers carry away each year. NASA's CALIPSO satellite, firing laser pulses through the atmosphere from 2006 to 2023, finally gave scientists the means to measure what sailors and Darwin had only glimpsed — a transoceanic nutrient cycle so exact in its accounting that it challenges the very idea of ecosystems as bounded, self-contained things. The Amazon is not merely a South Amer