a city built ON the ocean out of 50-ton basalt logs and we don't know how — Nan Madol

This is my favorite 'how did they DO it' that almost nobody knows. Nan Madol, off Pohnpei in Micronesia (Federated States of). ~90+ artificial islets in a lagoon, separated by tidal canals, so the whole city looks like it floats — 'Venice of the Pacific.' Built ~1200–1700 CE by the Saudeleur dynasty……