Nan Madol is an amazing ruin city of man-made canals and 92 islets. It was deservedly enlisted as a World Heritage Site in 2016. The temples, tombs and walls were made from huge basalt pillars and boulders, which were stacked upon each other in a clever interlocking way, so mortar wasn't necessary. The construction started in the late-1100s and was the ceremonial and political seat for the royalty of Pohnpei until the mid-1600s. Today, there is only one islet, Nan Dowas, left with erected walls, the other islands have just carved stones scattered around. Nan Dowas has some of the biggest stones, weighing up to 50 tons, and holds also a royal tomb and tunnels. You can walk to the islet at low tide, else you have to wade across the canals or getting a boat tour from kolonia.