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Penghu has two Tiezhan Shu Islands, named for their resemblance to iron anvils. The northern Tiezhan Shu, belonging to Basan Village, lies northwest of Penghu's main island, an uninhabited islet primarily composed of basalt tablelands and surrounded by steep cliffs. Its foundation is made of relatively fragile volcanic breccia underlying the columnar basalt. Influenced by the northeast monsoon, the northern coastline has been eroded over time by ocean currents, forming a massive sea cave that accommodates small boats. Larger than Xiji Island's Blue Hole, it is the largest sea cave in Penghu. Inside the cave, temperatures are three to five degrees cooler due to lack of direct sunlight, earning it the nickname "air-conditioned cave." After arriving on the island, visitors will see platform-like basalt formations, as well as distinctive honeycomb rock and basalt joints shaped by water erosion—all unique geological features of Northern Tiezhan Shu. This location was once used by the military as a target field, leaving behind numerous iron shrapnel and rock fragments. Due to the short, shallow intertidal zones and few platforms nearby, few people visit the island. Consequently, hundreds of birds nest there, especially white-rumped needletail swifts. When a boat approaches, the swifts emit warning calls and fly in flocks to the skies, seemingly reminding visitors that they are actually the rightful inhabitants here.