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Limestone is a sedimentary rock which consists of calcium carbonate and is usually formed from the shells of marine animals and calcium secreting plants. It is aggressively attacked by chemical weathering in humid climates. The process by which limestone is broken down is called carbonation, the dissolving of rock containing lime by rainwater which contains dissolved carbon dioxide percolating through the rock. The weak carbonic acid widens joints in the carboniferous limestone surfaces, producing bare limestone pavements where ridges alternate with grooves. Eventually sinkholes and underground caves are formed. Underground rivers form large underground cave systems. When the climate changes, the caves can be left dry.