Subterranean basalt megastructures of Nan Madol in the Pacific Ocean
Nan Madol is an ancient city in Micronesia built from about 750,000 tons of basalt on artificial islands atop a coral reef. Its biggest mystery is how a pre-industrial society transported and assembled the massive stones. While mainstream archaeology dates it to the Saudeleur Dynasty (c. 1100–1600 CE), some researchers argue that submerged foundations could be much older, fueling debate about the site’s true origins.