China is a country with many caves, especially those in Guangxi, such as Qixingyan in Guilin and reed flute cave. The Yunshui Cave near Zhoukoudian, a suburb in southwest Beijing, is 6 12 meters deep, and seven "halls" are connected by a long and narrow corridor. At the end of the cave is a huge stalagmite, which has the reputation of eighteen arhats. Behind stalagmites is an unfathomable sinkhole, which also has a certain scale. The keel cave in Zhoukoudian is not big, but it is the residence of our ancestors. Others include Yaolin Cave in Tonglu, Hangzhou, Shuanglong Cave in Jinhua, Zhejiang and Shanjuan Cave in Yixing, Jiangsu. All the rocks in the cave are limestone, and the composition of limestone is CaCO3. Calcium carbonate has such a property that when it meets water with dissolved CO2, it will become soluble calcium bicarbonate [Ca (HCO3) 2] CaCO3+CO2+H2O = Ca (HCO3). If the water in which calcium bicarbonate is dissolved is heated or the pressure is suddenly reduced, the calcium bicarbonate dissolved in water will decompose and become calcium carbonate again. Water drops fall from the top and deposit to form stalactites. Water drops drop down and deposit on the ground to grow up to form stalagmites, and stalactites and stalagmites are connected to form stone pillars.