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Before the elephants collapsed, they walked in aimless circles. Some fell head first, dying where they stood moments earlier; their carcasses scattered near watering holes across the Okavango delta. The unexplained deaths in May 2020 alarmed conservationists. By July, at least 350 elephants had died and nobody knew why.

The animals all had their tusks, so poaching was unlikely. A lot of them had obviously died relatively suddenly: they had dropped on to their sternums, which was indicating a sudden loss of muscle function or neural capacity," says Niall McCann, director of the conservation group National Park Rescue.