《水经注·卷三十七》
过去有人在冬天经过这里，把笠帽放在洞中，被风吸了进去，月余之后回来，沿着长杨溪步行，又检回那顶笠帽，可知洞与溪是暗通的。
往人有冬过者，置笠穴中，风吸之。经月还步杨溪，得其笠，则知潜通矣。
Someone in the past passed by here in the winter, and placed a hat in the cave, and it was sucked in by the wind; he came back more than a month later, and walked along the Changyang River, and picked up that same hat, which shows that the cave and the river are connected.