This freak of nature and ornithology is halfway between Jiànshuǐ and Gèjiù. The karst formations (the largest in Asia) are a lure, but what you'll want to see are the hundreds of thousands of swallows flying around in spring and summer. The cave is split into two – one high and dry, the other low and wet. The higher cave is so large that a three-storey pavilion and a tree fit inside.
Plank walkways link up; the Lú River runs through the lower cave for about 8km and you can tour the caverns in 'dragon-boats'.
There's no direct bus, but the ones bound for Méngzì, Kāiyuán or Gèjiù which don't take the expressway pass the cavern (¥10, one hour).
