Shidao

　　Shidao is situated over 180 nautical miles to the southeast of Hainan Island. It is the oldest of the Xisha Islands, with an age of at least ten thousand years. Shidao is also the highest island in the Xisha archipelago. Some say Shidao is the “pyramid” of the Xisha Islands. It’s called a pyramid not only because of its pyramid-like shape, but also due to some mysterious puzzles that echo the enigma of pyramids. Most islands in the Xisha Islands are formed from the accumulation of biological sediments such as coral and shells, resulting in relatively loose rock structures. However, Shidao is composed of certain types of layered bio-sandstone, giving it an exceptionally hard rock quality. Moreover, its structure differs from other layered bio-sandstones. Typically, in such formations, the lower layers are older than the upper ones—but Shidao is the opposite. This phenomenon remains scientifically unexplained to this day. Shidao is just a few kilometers away from Yongxing Island, with the two connected by an artificially built causeway. A large expanse of coral reef is enclosed between the two islands. At low tide, the seabed reefs can be clearly seen. From afar, it looks like a smooth, blue-green flatland, but the submerged reefs are actually rugged and jagged.