Er Wang Dong aka Second Royal Cave is a large cave in the Wulong Karst region China. Current known length of its passages is 42,139 metres with a maximum depth of 441 m. It is large enough to contain its own weather system. The cave starts in the 195 m deep Niubizi tiankeng (sinkhole) and also contains the 295 m 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, which contains one of the largest cave chambers in the world – Cloud Ladder Hall. Both caves were explored by Hong Meigui Cave Exploration Society.