Approximately 2 kilometers north of Dabaisha Islet (large Baisha Islet,) a distinct flat-topped island rises from the sea. This island, primarily covered with large, flat stone slabs, resembles the anvil used by blacksmiths—hence the name Tiezhanyu (North Anvil Island).The island stands about 5 meters tall, surrounded by sheer sea cliffs. At its highest point, the island features basalt formations shaped like an anvil, along with peculiar honeycomb rocks sculpted by natural erosion. During the 1960s, the island was once used as a military target range, leaving behind scattered fragments of metal and shattered rocks.
Situated off the northwest coast of Penghu's main island, Tiezhanyu is entirely composed of basalt. Its striking resemblance to an anvil is what inspired its name. Due to the relentless force of the winter northeast monsoon, the waves have carved a massive sea cave into the island's northern cliffs. This cave, exceeding 10 meters in height, is the largest in all of Penghu. Because of its unique position, it remains in perpetual shadow, making it significantly cooler than the surrounding area—even in summer. The temperature inside is typically 3 to 5 degrees Celsius lower than outside, attracting visitors looking to escape the heat. This cooling effect has earned the cave the nickname "Air-Conditioned Cave."
  
When viewed from the north at sea, the island's geological composition becomes strikingly clear: the upper section consists of solid basalt, while the lower portion is made of volcanic breccia—a much more fragile rock. Over time, the relentless winter waves have eroded the weaker volcanic breccia beneath the basalt, forming a vast sea cave. The cave is so expansive that small boats and fishing vessels can sail directly inside. Unlike other islands in the Penghu archipelago, Tiezhanyu has very limited tidal zones. The island lacks significant wave-cut platforms near the cave, with only a 5-meter-wide rocky platform on the eastern cliffs and small gravel beaches to the south and west. As a result, few local fishermen collect shellfish here, making it an ideal breeding ground for seabirds. Every year, over a thousand bridled terns nest on the island. As boats approach, flocks of these birds take flight, their calls echoing like the deep growls of small dogs. They then gather in large numbers above the visitors, circling in unison—a spectacular and mesmerizing sight.

Source: Penghu Travel