Nan Madol is a very special place made up of lots and lots of rocks in the middle of the ocean. It is kind of like a big city made of stones on the water! It was made a very, very long time ago by people called the Saudeleur who lived in Micronesia a long time ago.
Nan Madol is made up of lots of little islands called "islets" and they are all held together by the big rocks. It is like a puzzle where all the pieces fit together, and they have been made to stay together without falling apart for hundreds and hundreds of years. It is really impressive!
The stones that make up Nan Madol are very different from other stones because they are made of a special kind of rock called basalt. Basalt is a very hard kind of rock and is very difficult to work with. The Saudeleur had to work really hard to shape the rocks and make them fit together perfectly like a big puzzle. Even today, we don't really know how they did it because it was so difficult.
There are lots of stories and legends about Nan Madol. Some people believe that it was built by magic because it is so amazing and impossible. Others think that it was used for religious ceremonies or to show how powerful the Saudeleur were. Nobody really knows for certain why Nan Madol was built, but everyone agrees that it is a very special and important place.