Tiankeng

Tiankeng, a super-large collapsed doline, is formed by the collapse of large underground river hall. It is usually developed in the carbonatite stratum with huge thick vadose zone, stretching from underground to surface, with its plane width and depth up to more than one hundred meters. Having steep and enclosed palisades and deep well-shape outline, it connects to underground river at the bottom (or there might have underground river before). The ones losing underground river track or whose peripheral wall was damaged can be called degenerated tiankengs.
 
Daluoquan (Left) and Xiaoluoquan tiankengs (Degenerated collapsed tiankeng)