Nan Madol
With no draft animals, no metal tools, and no wheel, how did a Pacific island society quarry, move, and stack tens of thousands of multi-tonne basalt columns across open reef water to build a city of nearly 100 artificial islets — and why, after roughly four centuries as the seat of Saudeleur power, was it abandoned?
c. 12th-13th century CE monumental construction · Temwen Island, Pohnpei, Federated States of Micronesia