Xiaozhai Tiankeng sinkhole

The largest sinkhole of the world is Xiaozhai Tiankeng - incredible, up to 662 m deep pit with nearly vertical walls. This is one of the most impressive natural attractions on Earth.

Tiankeng is giant sinkhole with vertical walls, at least 100 m deep and 100 m wide. Most often bottom of tiankeng is crossed by underground river, which is open to daylight only here.

http://www.wondermondo.com/Countries/As/China/Chongqing/Xiaozhai.htm