French Polynesia

A volcano two million years past its peak, sinking slowly back into the Pacific and taking its coastline with it. What is left above water are green ridges rising almost vertically out of a lagoon — and what was once the shoreline is now a ring of reef a kilometre offshore.

Between the reef and the island the water is shallow, still and lit from below by white sand, running through every blue that exists. Given another million years the peaks will go under and the ring will remain, an atoll around nothing at all.