Nan Madol, the city on the reef

A real megalithic city, and the engineering is genuinely unexplained

Nan Madol lies in a lagoon against the eastern shore of Pohnpei, in the Madolenihmw district of the Federated States of Micronesia, between the main island and Temwen Island. Its stone-walled core, about 1.5 by 0.5 kilometres, holds 92 artificial islets, platforms of stone and coral fill bordered by tidal canals, and UNESCO counts more than 100 islets across the whole property; the archaeological district covers more than 18 square kilometres. The name means within the intervals, for the canals that cross it, and it has been called the Venice of the Pacific.

People were active here by the first or second century AD, and islet building had begun by the 8th or 9th century. The megalithic work began around 1180, when great columnar basalt stones were brought from a volcanic plug on the far side of Pohnpei and laid in a distinctive header-stretcher technique; UNESCO dates the palaces, temples, tombs and dwellings to 1200 to 1500. This was the ceremonial and political seat of the Saudeleur dynasty, which united an estimated 25,000 Pohnpeians until about 1628, when Isokelekel overthrew it. The Saudeleurs kept potential rivals close by making them live here, and food and water had to be brought by boat, since the islets have neither.

Pohnpeian legend gives the building to twin sorcerers, Olisihpa and Olosohpa, who came from the west in a great canoe looking for a place to worship Nahnisohn Sahpw, the god of agriculture, and levitated the stones with the help of a flying dragon. Olosohpa became the first Saudeleur. The royal mortuary islet of Nandauwas, built for him, is walled 5.5 to 7.5 metres high around a central tomb. The mortuary sector, Madol Powe, alone has 58 islets.

The city itself was probably home to no more than 1,000 people, perhaps half that, and after the Saudeleurs fell their successors gradually abandoned it; residential use had ended by the 1820s. UNESCO listed Nan Madol in 2016 and put it straight on the List of World Heritage in Danger: silt is choking the canals, mangroves are spreading unchecked, and walls are collapsing and disappearing under overgrowth. It is traditionally owned and protected by the Nahnmwarki of Madolenihmw, and the Park Service notes that the canals are still stable enough for visitors to tour it by boat.