Nan Madol, on the island of Pohnpei, came as a wonderful surprise to me as part of my trip flying on United's Island Hopper flight between Honolulu and Guam. It was a small city, constructed of stone on artificial islets along the coast of Pohnpei. Construction is dated to the late 1100s and it served as the capital of the Saudeleur Dynasty of the islands until 1628. The scale of it all is truly amazing. Great stone buildings separated by a series of ocean canals, protected by a large breakwater wall. The most impressive remnant is the tomb complex in the center, where the building's (palace's?) remaining walls are over 20 feet tall in spots. And what incredible walls these are! It is constructed of a mix of massive basalt boulders and huge hexagonal basalt crystals. You might be familiar with these hexagonal crystals from the Giant's Causeway in Northern Ireland. Here, long pieces of them are stacked to form walls, reminding me of a set of heavyweight Lincoln Logs, stacked in bunches to form these formidable walls. But the mystery of how this small island civilization constructed all this gets even better, because this type of basalt rock and crystal form is only found on the opposite side of the island! Some of these stones weigh many tons each. The local legends say that the stones were magically flown through the air from the other side of the island to build this city. However they managed to accomplish it, I am very impressed. It was really exciting to encounter such an archaeological mystery that I had never heard of before this trip.
After visiting Nan Madol, we also stopped at the Pohnpaip Petroglyphs which are one the way back to town. These were pretty nice to visit and you get to walk right out among them (they are carved into a house-sized rock outcropping). Sadly, it didn't seem like anyone really know anything about their historical context or meaning at all.
A fun side note: the taxi driver who drove us to a restaurant one night told us that his father's family was the one who owned this part of the island and so he lived near there growing up. He told us some of the stories he'd been told as a boy, including repeating the "stones flying through the air" part, and that no one ever goes into the complex at night now, that that is completely forbidden.