Have you ever seen a blacksmith’s anvil? Two Penghu island’s are named for their anvil-like appearance. Beitiezhenyu Island, “north anvil island” is in Baisha Township, northwest of Penghu Island. Steep sea cliffs border the isle’s uninhabited basalt mesa; the columnar basalt's lowest layer is relatively fragile volcanic breccia. Driven by the northeast monsoon winds, sea currents eroded a massive cave on the north shore that’s big enough to accommodate a small boat––it’s even bigger than Xijiyu’s “blue sea cave”; in fact, it’s the largest marine erosion cave in Penghu. The sun's rays never reach the interior, so the temperature in the cave is three to five degrees lower than the outside. That's why people call it the "air-conditioned cavern."

After landing on Beitiezhenyu, look off into the distance. You’ll see basalt platforms, unusual honeycombed rock formations, and basalt joint grooves shaped by marine erosion – the islet’s distinctive topographical features.

The national military once used Beitiezhenyu as a target practice range; scattered iron and rock fragments are a reminder of that time. The intertidal zone is short and shallow; moreover, there are few surrounding sea platforms, so few people come here. Consequently, birds in the thousands gather on the island to roost and breed. The white-browed tern is the most common. When boats approach, the terns make warning sounds, flock into the sky, and hover overhead. It's as if they're reminding tourists that birds are the masters here!