Egg shells are almost pure calcium carbonate. CaCO3 can also be found in the shells of many marine organisms and in both limestone and marble. The fact that none of these substances dissolve in water suggests that CaCO3 is normally insoluble in water. Calcium carbonate will dissolve in water saturated with CO2, however, because carbonated water (or carbonic acid) reacts with calcium carbonate to form calcium bicarbonate, which is soluble in water.
CaCO3(s) + H2CO3(aq) Ca2+(aq) + 2 HCO3-(aq)
When water rich in carbon dioxide flows through limestone formations, part of the limestone dissolves. If the CO2 escapes from this water, or if some of the water evaporates, solid CaCO3 is redeposited. When this happens as water runs across the roof of a cavern, stalactites, which hang from the roof of the cave, are formed. If the water drops before the carbonate reprecipitates, stalagmites, which grow from the floor of the cave, are formed.
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch10/carbon.php
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