The Qing River water contains a lot of carbon dioxide—basically carbonated water—which dissolves limestone, turning it into soluble calcium bicarbonate. Before the underground river changed course, whenever the Qing River flooded, the fast-moving, high-volume water would carry sand and gravel, scrubbing the cave walls and entrance like sandpaper. This erosion widened and deepened the cave opening over time. Eventually, when the limestone got so dissolved that it couldn’t hold its own weight, sections would collapse, making the cave entrance even bigger...