由于地下伏流的侵蚀，洞穴发育并伴随崩塌。地下河水不断搬运大量的崩塌堆积物，洞穴空间逐渐扩大。随着地下河的继续发育，顶板因局部崩塌形成竖井状垂直洞道，甚至塌至地表，从而形成天窗。九九洞天为天窗，是地下伏流“通天河河道”的透光部位，向上连通到地表。洞道蜿蜒曲折，长约99m，通至山顶。 Due to the erosion of the subterranean river, the cave was developed with rockfalls. The underground river constantly carried a lot of collapse deposits, and the cave space gradually expanded. As the underground river continued to develop, a vertical shaft was formed by partial collapse of the cave ceiling. When the ceiling collapsed, a karst window was formed. The Deep cave karst window is the light-through part of the subterranean river Tongtian river watercourse and connects up to the surface. The tunnel wanders to the top of the mountain, about 99m long.