Drop a master builder on the southeast part of the island, between the food bin and the stone temple. It will probably take a few tries, but keep up the work until, in the bar below "Action", it says "Is something buried here?". The builder will start digging until a construction site appears. It will take a while, but eventually a treasure chest will appear.
Using advanced building techniques and scientific methods of detection, one of your villagers uncovered a buried treasure!