On the island of Pohnpei, Micronesia rest the remarkable ruins of Nan Madol, the only ancient city ever constructed on top of a coral reef. Referred to as an “engineering marvel” by the Smithsonian and nicknamed the “Venice of the Pacific,” this series of over 90 artificial islets could have once housed around 1,000 people. Although the Saudeleur built the city around 1200 AD, it wasn’t until earlier this year Nan Madol was finally named a World Heritage Site.

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Nan Madol flourished sometime during the 13th to 17th centuries AD as a spiritual and political center for the Saudeleur. Little remains of the intriguing ancient civilization – no art or carvings – other than marvelous ruins atop the coral reef. Oral history says the Saudeleur came to Pohnpei as foreigners in 1100 and ended up ruling the island, with Nan Madol as their dynastic seat. The city also served as a temple for the god the nobility worshiped.

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The Saudeleur utilized columnar basalt, a kind of volcanic rock, to build the impressive city on a foundation of coral – and as the building materials are so heavy, no one has yet figured out how they accomplished the feat. The heaviest pillars weigh around 100,000 pounds. The walls surrounding the island’s largest structure, a royal temple called Nandauwas, are 25 feet high.

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The enduring stability of the remains is also something of a mystery. According to the National Park Service, “The Pohnpeians, who had neither binding agents like concrete nor modern diving equipment, sank the heavy stones into the lagoon using an unknown method. The building remains and canals are stable enough that even after centuries of abandonment visitors can still tour Nan Madol by boat.”

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Earlier in 2016, the World Heritage Committee added Nan Madol to both the World Heritage List and the List of World Heritage in Danger, underlining the need to protect the fascinating site from unchecked mangrove growth and waterway siltation. Nan Madol is Micronesia’s first World Heritage Site.

Via Smithsonian.com, Metropolitan Museum of Art, and National Park Service

Images via Stephanie Batzer on Flickr (1,2,3,4), Stefan Krasowski on Flickr, and Wikimedia Commons (1,2)