A tiankeng is also known as a giant collapsed doline or sinkhole although the word is Chinese for sky hole or heavenly pit. The name was proposed by a Chinese geologist in the 1990s, since there was no terminology for the unique geological phenomenon. Giant collapsed dolines are defined as tiankengs if they are more than 100m deep and wide. However, they are defined as giant tiankengs if they are more than 500m deep and wide. In general, the majority of tiankengs are found in the karst regions of the drainage basins of the Yangtze and Peal rivers in southwestern China. These areas are very remote and therefore it was not until the mid 1990s that the tiankengs were discovered and received their unique name.
With a depth of 662m, a width of 527m and a 119 million cubic meter capacity Xiaozhai tiankeng is the largest giant tiankeng on the planet. Its enormity is quite hard to imagine (here you can get an impression of just how big Xiaozhai is compared to a Boeing 747: http://bit.ly/183oD1W) The doline, located in the Chongqing Municipality of southwest China was discovered by scientists in 1994. Among locals the doline has been known since ancient time, Xiaozhai is actually the name of an closeby abandoned village and translates as little village.
Xiaozhai consists of two so-called steps: an upper doline of 320m depth and a lower of 342m depth with vertical walls which are separated by a sloping ridge. During rainy weather these steps display a waterfall. The tiankeng formed several 10,000 years ago after the collapse of lower Triassic limestone Difeng cave which in turn is due to karst processes in carbonate (in this case very pure limestone) rocks. Underneath the sinkhole the underground Migong river flows for more than 8km during which it drops elevation 364m. The bottom of the tiankeng is the only location where the underground river shows itself.
In addition, Xiaozhai Tiankeng displays a rare biodiversity with over 1285 plant species and animals such as the clouded leopard walking around the bottom of the pit. For another good impression of the enormity of this doline and extreme base jumping you could do here, watch the following video http://bit.ly/13z7igf. (Beware of and also annoy yourself with energy drink promotion):
Image: http://bit.ly/1071M3I. Unfortunately a good impression of the scale of Xiaozhai tiankeng is hard to find.