Groundwater can cause erosion through chemical weathering. First, water that sinks into the ground mixes with carbon dioxide to form carbonic acid, a weak acid. Carbonic acid can break down limestone. Some of the limestone changes chemically and is carried away in a solution of water. This gradually hollows out pockets in the rock. Over time, these pockets develop into large holes underground—caves or caverns.
Rainwater picks up carbon dioxide from the air and as it percolates through the soil, which turns into a weak acid. This slowly dissolves out the limestone along the joints, bedding planes and fractures, some of which become enlarged enough to form caves.
