Nan Madol, the city on the reef

Ninety-two artificial islets built out into the lagoon off Pohnpei, floors of coral and stone fill walled with columnar basalt, threaded by tidal canals you move through by boat. The basalt came from a volcanic plug on the far side of the island; nobody has satisfactorily explained how a society of under a thousand people moved that much stone that far. The core alone is about 1.5 by 0.5 km and the wider district covers more than 18 square kilometres. People were here by the first or second century AD, islet building started around the eighth or ninth, and the megalithic work went up from about 1180. It was the ceremonial and political seat of the Saudeleur dynasty until roughly 1628. UNESCO World Heritage since 2016. Federated States of Micronesia.