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Rainwater absorbs carbon dioxide from the atmosphere as it falls through the air. The carbon dioxide combines with water to form carbonic acid. The slightly acidic water sinks into the ground and moves through spaces in soil called pores. The acidic water also travels downward through cracks in rock. Water that flows under the ground through soil or rock is called groundwater. Groundwater is a strong erosional force. Working slowly over many years, groundwater moving through cracks in rocks dissolves and carries away rock minerals. This enlarges the cracks little by little. Eventually a cave may form.
Dissolved minerals in groundwater may also be deposited in a cave, creating formations called stalactites and stalagmites. They form when calcium carbonate dissolved in groundwater comes out of solution. Stalactites form as water drips from the ceiling of a cave, creating icicle-like formations. Stalagmites form as water drips onto the floor of a cave, creating formations like upside-down icicles that grow from the floor of the cave upward. If a stalactite and stalagmite join together, they form a column. Sometimes the roof of a cave collapses. This forms a hole on the surface called a sinkhole. A sinkhole may be large enough to swallow a home or even several homes.
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