The Luobi Cave is a karst cave, meaning it's made of soluble rocks and has a lot of sinkholes and imposing stalactites. It's impressive, with the entrance almost 40 feet high and the apex inside about 60 feet high. This is truly a natural cathedral.
Near the entrance, there are two big stalactites that look like large pens, giving the cave its rather boring name ("Two Pens"). It's a protected historic site, with carvings inside dating as far back as the 1200s. Even cooler, the cave was used during the Upper Paleolithic era (10,000 years ago), and there's evidence of the earliest humans in Hainan.