Er Wang Dong, « la seconde caverne royale » (Chine)

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Er Wang Dong (二王洞; literally: « Second Royal Cave ») is a large cave in the Wulong Karst region, in Wulong County of Chongqing Municipality of China.
Current known length of its passages is 42,139 metres (138,251 ft) with a maximum depth of 441 m (1,447 ft). It is large enough to contain its own weather system. The cave starts in the 195 m (640 ft) deep Niubizi tiankeng (sinkhole) and also contains the 295 m (968 ft) deep Qingkou tiankeng.
Er Wang Dong cave has formed in Lower Ordovician limestone and is located close to another very large cave system – San Wang Dong.
Both caves were explored by Hong Meigui Cave Exploration Society. Local people make weather forecast based on the observations in Niubizi tiankeng – if there is a fog coming from the cave, rain is expected. (from Wiki En)