If heading 2km northward from Big Paisha Island, there is an island with typical mesa terrain. The island, mainly composed of giant slates, is smooth and looks like a board used for a blacksmith to strike the iron. Therefore, the island is called Tiejhanyu. The island, with vertical cliffs all around, is around 5 meters tall. In addition to board-shaped basalt rocks, there are also lots of honeycomb rocks in interesting shapes on the island's highest spot. Since the place was used as a shooting range of the national armed forces in the 1960s, many iron chips and rock pieces can be found as well.
Tiejhanyu is an offshore island located in the northwest of Penghu’s main island. The whole island is composed of basalt and got its name because of its shape similar to a board.
Waves caused by the northeast monsoon in winter have hit the north shore of Tiejhanyu for a long period of time and further formed a 10-meter-long sea cave, which is the biggest sea cave in Penghu. The sun can’t shine through due to the unique position of the cave, making the temperature of the cave 3-5 Celsius degrees lower than the outside even in summer. Visitors are attracted to the cave to escape the heat and relax, and with the environment as cool as a room with an air conditioner turned on, the cave is called Air Conditioning Cave.
If looking at Tiejhanyu from the northern sea area, one can clearly find that the upper part of the island cliff is basalt while the lower part is volcanic breccia. The volcanic breccia is relatively fragile and eroded by the winter waves, forming a giant sea cave where small yachts and fishing boats can enter at the bottom of the northern cliff. The intertidal zone of the island is rather shallow, and abrasion platforms can hardly be found around the sea cave, except for a platform 5 meters wide under the eastern cliff. In addition, there are only small gravel beaches in the south and the west. With such an environment, only a few fishermen collect conches and shellfish, which attracts more than a thousand bridled terns to breed on the island every year. When boats get close to the shore, flocks of bridled terns fly up from the island, give deep alarm calls that sound like the barks of puppies, and hover over the boats at last, which is a stunning view on the island.