Limestone rock is composed of organic calcite. Calcite is a carbonate mineral that is highly reactive with acid. As a result, limestone rock is also highly reactive with acid. Acids are found almost everywhere on Earth. The most common and abundant source of acid in nature is rainwater. Rainwater becomes slightly acidic when carbon dioxide and other gases absorb into rain and clouds. When this acidic rain reaches the surface of the Earth, it is capable of chemically weathering the rocks exposed at the surface and the rocks located below the surface. When an area has limestone present, rain is capable of weathering rock in a way that creates intricate underground holes that are called caves and caverns.
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