By some accounts, French Polynesia is heaven on Earth. It’s a collection of 118 tropical islands and atolls spread across a wide expanse of the Pacific. (Perhaps you’ve heard of Tahiti.) And yet, thanks to erosion, land subsidence, and sea level rise, parts of this paradise are threatened with sinking beneath the waves.

“When we looked at it, the options were pretty bad,” said Marc Collins Chen, who in the 2000s was the minister of tourism. “One was to retreat. The other was to build large engineering public works, like dikes and seawalls.”

That’s the choice a lot of low-lying places are facing in the coming century. For some, like Miami or Bangladesh, retreat is at least possible. Island nations, Chen realized, had nowhere to turn and needed another option. Chen had one: “To have entire floating cities.”

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