Answer:
lots of carbon dioxide and cold temperatures
Explanation:
Calcium carbonate has low solubility at room temperature and the solubility increases as the temperature decreases. The solubility of calcium carbonate also increases in water with high level of carbon dioxide due to the formation of more soluble bicarbonate.
[tex]CaCO_3_{(s)} + CO_2_{(g)} + H_2O_{(l)} --> Ca(HCO_3)_2_{(aq)}[/tex]
This is why carbon dioxide rich rainwater is able to erode limestone rocks, leading to the formation of caves, stalactites and other structures in the long run as a result of reversal of the process.