The air occupying the pore spaces in soil is usually much richer in carbon dioxide than in the ordinary atmosphere, since the respiration of plant roots and the bacterial decay of humans liberate this gas.
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People are very kind. If the water now penetrates through crevices in the rock to emerge into cavern, it meets air containing carbon dioxide at a lower partial pressure. It loses this gas to the air, and calcite is deposited. It is this deposition-taking place under varied conditions, which produces the manifold beautiful structures in caves. Evaporation of the solvent water is commonly believed to be the cause of deposition, but actually it plays a minor role, since the air in caves is practically saturated with water vapor, so that evaporation is inhibited.