Sandun

　　(Islands Linked as a Gem) Sandun, situated 10 kilometers off the coast south of Xu Town, is made up of three small islets, namely First Mound, Second Mound, and Third Mound, collectively referred to as “the Three Mounds.” Nearby there is also a small sandy mound called Lou Shi Mound. First Mound is the closest to the shore and can be reached on foot during low tide, followed by Second Mound, while Third Mound is the farthest and requires a boat to access. The mounds feature remnants of ancient temples and wells. During the Qing Dynasty, people poetically named the well there “Dragon Spring and Clear Well.” The Three Mounds are interlinked, standing in the strait, presenting an extraordinary scene, elegantly referred to as “Islands Linked as a Gem.”

　　On the southern beach of Er Dun Island, there is an expanse of exposed basalt boulders. These massive rocks were formed when hot lava erupted or flowed out onto the surface and was suddenly cooled by seawater. The intense temperature gradient caused the surfaces of the rocks to contract unevenly, forming patterns resembling turtle shells—strikingly lifelike, truly a remarkable spectacle.

　　Currently, the access trestle bridge of the Guangdong–Hainan Railway has been built here, and the west breakwater runs through First Mound and Second Mound, connecting the islet islands to the mainland, allowing visitors to reach them by vehicle. On Second Mound Island, there is a 40-meter-high modern navigation lighthouse where visitors can climb to enjoy a panoramic view, watch the tides, and listen to the waves.