The Swallow Grotto (Yanzikou) trail is located at 179 km along the Central Cross-Island Highway. The trail stretches from Jinheng Bridge in the west to Zhuilu Cliff on the other. It is 1.37 km in length， and takes about 30 minutes to traverse on foot. It is the finest section of the entire Taroko.Walking along the Swallow Grotto， you will see the gorges， natural sinkholes， water geysers， and the uniquely shaped "Rock of the Indian Chief". The reason behind these geographic forms is the river. This part of the river valley is extremely narrow and deep， therefore as the river comes through ， a formation known in Chinese as "The one thread of Heaven" is created. This is a cave or a canyon with a tiny crack or opening at the top， which when viewed from below looks like a narrow thread of sunlight. When the water runs through the canyon， it speeds up due to the narrow channel， creating a much more potent erosive force on the marble embankments， and carves out the beautiful and unique natural forms of Swallow Grotto. From the Lookout， you can listen to the sound of the rushing river and admire the marvelous cliff and rock formations.