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The outer sandbar is located about 10 kilometers off the coast of Yunlin and Chiayi, with a current area of approximately 1,000 hectares. It is named for its shape resembling an umbrella and is Taiwan's largest sandbar. The outer sandbar is affected by waves, coastal currents, and the northeast monsoon, resulting in an average annual southwestward drift of 60 to 70 meters, earning it the reputation as "moving national soil." It is primarily formed by the accumulation of large amounts of sediment carried by the Zhuo-Water溪 River. The presence of the outer sandbar has created a calm and vast sea area between the sandbar and Chiayi County, known as the Dongshi tidal flat, which has led to the thriving of oyster cultivation. In recent years, due to the reduction of sand sources, the area of the outer sandbar has gradually decreased. Relevant authorities are making efforts to rescue the disappearing outer sandbar, and many tourists have come to witness its beauty. The Wenhafen Stack Lighthouse is the only lighthouse on the outer sandbar, with a steel frame tower structure. The construction of the Wen-hafen Stack Lighthouse was due to the approximately 5-kilometer-long and 1.2-kilometer-wide floating sand dune where it is located, which often appears and disappears with the tides, affecting the passage of nearby waterway vessels. For safety reasons, the lighthouse was built. However, since the Wenhafen Stack Lighthouse was built on the sandbar, it has continuously drifted and faced the risk of its foundation being eroded. The Wenhafen Stack Lighthouse has been rebuilt seven times, setting a record for the highest number of rebuilds of a lighthouse in Taiwan and worldwide.