In the warm waters around Guam, a coral reef quietly recorded a series of events taking place thousands of kilometres away. During the 1950s, the United States carried out powerful thermonuclear tests at Bikini and Enewetak in the Pacific. Decades later, scientists studying a coral core from Guam found that its annual growth had preserved distinct carbon-14 changes corresponding to three of those testing campaigns. The strongest signal followed Operation Castle in 1954, while smaller rises appeared after Operation Redwing in 1956 and Operation Hardtack I in 1958. The pattern was more than a general trace of the nuclear age. The timing and shape of the peaks showed that material from the testing grounds had entered the ocean and travelled towards Guam with Pacific currents. The coral had effectively kept a chemical record of that journey, allowing scientists to reconstruct how the signal moved through the western Pacific.
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