What is the meaning of "tiankeng" in folk customs?

In folk customs, "Tiankeng" refers to the super-large karst negative terrain with extraordinary spatial and morphological characteristics such as huge volume, steep and trapped rock wall, deep well-shaped or barrel-shaped outline. Tiankeng usually develops in the soluble rock stratum with extremely huge thickness and extremely deep groundwater level, which leads from underground to the ground, with an average width and depth exceeding 100 meter, and the bottom is connected with the underground river (or there is evidence that the underground river has moved).