Cave explorers stumbled upon a prehistoric forest at the bottom of a giant sinkhole in South China earlier this month. Sinkholes such as these are also known in Chinese as Tiankeng, or “Heavenly pit.”  At 630 feet deep, the sinkhole would hide the Washington Monument and then some. The bottom of the pit holds an ancient forest spanning nearly three football fields in length, with trees towering over 100 feet high. And according to the Chinese government, it is one of 30 enormous sinkholes in the county.  …Karst is a type of topography, ideal for geological wonders like the sinkhole in Leye County, created by groundwater dissolving the limestone rock beneath the surface, according to the U.S. Geological Survey.   …About 13% of China is covered by karst topography, according to NASA, with the Guangxi region being a prime example of its beauty.