The pH of rainwater runs about 5.5, due to the CO2 in the air. Limestone caves can result from the slow dissolution of calcium carbonate in buried strata due to the percolation of this slightly rainwater through the overlying earth. Anyone who grows a garden in a damp climate knows that you have to add limestone to the soil periodically to keep soil pH up.

The solubility of calcium carbonate is influenced by several factors, and they are all in play at once. A major one is acidity. The ratio of carbonate to bicarbonate in solution shifts toward bicarbonate as acidity increases (pH falls). Solid calcium carbonate in contact with supersaturated carbonate solution will see no net dissolution (although there will be microscopic interchange of carbonate at the surface). Lower the pH and the ratio of carbonate to bicarbonate falls. If it drops far enough then the calcium carbonate begins to undergo net dissolution.

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The Devil offered me power. I told him I preferred aperture.