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The volcanic geology of the Pescadores Islands (Penghu) is mainly basalt, with the exception of Hua-Yu Island, which is composed of andesite. Andesite forms from rapidly cooled lava expelled during volcanic eruptions, is more viscous, flows slower, and tends to accumulate in conical shapes near the volcano's vent, differing from the flat-topped table mountain formations of basalt. Formed approximately 65 million years ago, Hua-Yu Island is the oldest and furthest western island in Penghu. Located northwest of Wang-An, Hua-Yu's name derives from its numerous native plant species, over a hundred kinds of indigenous flora. The island today has more than one hundred residents who mostly rely on fishing due to its rich harvests, as it lies at the confluence of the Kuroshio Current and its branch with the Oyashio Current. Yan-Den Mountain in the east rises to 53 meters, the island's highest point, while the western mountain top hosts Hua-Yu Lighthouse—a white concrete tower considered the westernmost lighthouse in Taiwan and a key landmark. On the southeastern rocky outcrops stand the legend-enshrined "Immortal's Footprint" (left foot) of Lü Dongbin; it pairs perfectly with the right footprint on Wang-An Island's Tiantai Mountain, but visibility occurs only during low tide!