Phan Yod Stone Castle is a newly discovered tourist attraction in Satun Province, recently opened for exploration. Here, you can witness the beauty of caves that are over a million years old. In fact, Phan Yod Stone Castle is located on Koh Khao Yai. Millions of years ago, groundwater eroded and dissolved the limestone underground, creating numerous cavities. Over time, these cavities expanded and interconnected to form larger voids. Areas where the limestone was easily dissolved became depressions, while harder rock formations took shape as sharp pinnacles. Eventually, this process resulted in a sinkhole within the small mountain, leading to the present-day site of Phan Yod Stone Castle.