Nan Madol is a city of stone built on a coral reef off the island of Pohnpei, in the Federated States of Micronesia, and nobody carved any of it. The walls are basalt prisms, and Pohnpei's volcanic rock makes them that shape on its own: a thick lava flow contracts as it cools and cracks into regular five- and six-sided columns. The builders only had to prise the rods loose from flows on the far side of the island and stack them crosswise, like the logs of a cabin, with no mortar anywhere. Nearly a hundred artificial islets went up this way on the reef flat, coral rubble packed inside walls of black column, canals where streets would be. How they moved stones of many tonnes across the island without wheels or draft animals has never been explained.
You reach the islets by small boat across a still green lagoon. The channels are narrow, and the black walls slide past on either side under the mangroves. When the tide falls the channels drain and you can wade between them instead, coral crunching underfoot and the walls closing in on you. After one of Pohnpei's near-daily showers the stone turns glossy and mist lifts off the water. Nandauwas is where the scale lands on you: the royal mortuary and grandest islet, ringed by stacked columns rising over seven metres, with a court inside where the Saudeleur kings were entombed. That dynasty ruled from here until its god-kings grew harsh enough to be brought down. A warrior named Isokelekel, said to be the son of a thunder god, came with a company of fighters and ended the line, and the islets emptied. Nobody has lived among these stones for four centuries, and the reef has been taking its capital back, mangrove roots prising the columns apart. Pohnpeian tradition does not find the engineering hard. Two sorcerer brothers sailed in from a land to the west looking for somewhere to worship, and they flew the columns into place through the air with a dragon's help.