Maoyu is 16 kilometers southwest of Wang'an and 8 kilometers south of Huayu. It's part of Huayu Village, Wang'an Township, comprising Big Maoyu, Little Maoyu, and surrounding reefs. Both the larger and smaller islands are composed of basalt and surrounded by steep cliffs.

Big Maoyu is a giant geological dike. It’s the highest point in the Penghu islands, towering at 70-79 meters above sea level. It’s a textbook example of topography shaped by marine erosion. Sea caves abound, and there's a massive eroded arch on a sea cliff at the north end. An erosion platform on the east is exposed at low tide, revealing many basalt potholes. Little Maoyu is the east of Big Maoyu, with even steeper cliffs. A vein stone composed of volcanic agglomerates interlinks the two islands at low tide.

“Niaoyu has no birds, and Maoyu has no cats” (the former translates as “bird island” and the latter as “cat island”). Maoyu is so named because it looks like two squatting cats, not because cats abound––and can even said to be uninhabited. But lots of migratory birds make their home here. Terns––mainly white-browed terns and black terns––come here in summer to breed, favoring Maoyu’s steep, barren terrain. Indeed, three to five thousand terns mate on Maoyu each year. If you want to watch black terns in Penghu, Maoyu is the place to come––they appear sporadically in the northern islands, as well as the occasional Noddy Tern. Therefore, in 1991, the Penghu County government declared Big Maoyu and Little Maoyu a "seabird sanctuary." The protected area covers the land and extends 100 meters out from shore. Landing on the islands is forbidden under all but special circumstances.