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Researchers have restored1 circulation and cellular activity in the vital organs of pigs, such as the heart and brain, one hour after the animals died. The research challenges the idea that cardiac death — which occurs when blood circulation and oxygenation stops — is irreversible, and raises ethical questions about the definition of death. The work follows 2019 experiments2 by the same scientists in which they revived the disembodied brains of pigs four hours after the animals died, calling into question the idea that brain death is final.