Kariba’s difficulties are more complicated. It has been nearly incapacitated by ongoing drought, which has lowered the reservoir’s volume to twelve per cent of its usual capacity. But if the reservoir is refilled, the dam faces the possibility of collapse. It was built in the late nineteen-fifties, and in the years since water flowing through the dam’s six floodgates has carved a three-hundred-foot-deep pit, or plunge pool, at its base. The plunge pool extends to within a hundred and thirty feet of the dam’s foundation; if it reaches the foundation, the dam will collapse. That seems hard to imagine now, with the reservoir at a record-low level. But the Zambezi River Basin, on which the dam sits, is the most susceptible of Africa’s thirteen basins to exceptional droughts and floods, and climate change is intensifying both.