Nan Madol is about a hundred artificial islets raised on a reef flat off Temwen Island in Pohnpei, walled with columnar basalt, naturally hexagonal prisms formed in cooling lava, stacked in crossed courses like log-work, and separated by tidal canals. Construction ran from roughly the twelfth to the seventeenth century. The royal mortuary enclosure at Nandauwas has walls reaching about seven and a half metres, and the total quantity of stone moved runs to hundreds of thousands of tonnes.
It was the seat of the Saudeleur dynasty, which ruled Pohnpei from it and required subject chiefs to reside there where they could be watched, the arrangement being, among other things, a device for political control. The complex divides into Madol Powe, the mortuary and priestly sector, and Madol Pah, the administrative and residential one.
Pohnpeian tradition gives an account of its own: the brothers Olisihpa and Olosohpa founded the place, the stones were moved by sorcery, and the Saudeleur line was eventually overthrown by Isokelekel, from whom the island's later title-holders descend. That title system remains in use on Pohnpei, and much of Nan Madol stands on clan-held land, which is why its management is not a straightforward heritage question with a single authority able to answer for it.
The site has attracted a persistent body of invented history, and it deserves a plain answer rather than a hearing. Nan Madol has been claimed as a remnant of a drowned continent, Mu or Lemuria, and as evidence of construction by non-human visitors. Both ideas were manufactured in Europe and North America in the nineteenth and twentieth centuries. Mu was the invention of James Churchward, who produced no evidence and claimed sources nobody has seen; Lemuria began as a discarded nineteenth-century zoological hypothesis about animal distribution and was taken up by occultists. Neither has any archaeological, geological or genetic support, and plate tectonics leaves no room in the Pacific for a sunken continent.
What the evidence shows is that the basalt is local, quarried on Pohnpei; that the technique is Pohnpeian; and that the logistics (rafts at high tide, levers, ramps, and a chiefly system capable of commanding sustained labour), are well within the reach of the island's population. The persistence of the alternative claims has a familiar shape: they arise where impressive stonework is found among people whom the claimants did not expect to be capable of it, which is the same reflex that produced the Great Zimbabwe theories.
UNESCO inscribed Nan Madol in 2016 and placed it on the List of World Heritage in Danger in the same act, for silting canals and mangrove growth prising the walls apart.