Demonstration of the insolubility of calcium carbonate. Calcium carbonate is a very common component of minerals, being found as chalk, limestone and marble amongst others. It is hardly soluble at all in pure water, as seen here, where even stirring fails to make much difference. However, importantly, it is much more soluble as the acidity of the solution increases. As rainwater has a naturally slightly lower pH than pure water (due to dissolved atmospheric carbon dioxide), limestone readily dissolves away over geological timescales. The majority of the world's large cave systems are found in limestone regions for this reason.