Have you ever been in a cave before? What comes to mind when you hear the word? Have you ever wondered how caves are formed? Depending on what kind a cave is, it could have been formed by water erosion, chemical processes, or even by molten rock from a volcano. There are sea caves and other caves hollowed out by erosion from water pressure, and volcanic caves formed by empty lava tubes.

And then there's the other way for caves to be made: acidic water trickling through cracks in rock will gradually dissolve limestone and similar rocks. Carbon dioxide from rotting plants in soil, and sulfuric acid from underground gases or acid rain, can combine with water to make an acidic solution that will dissolve limestone. Gradually, this acidic water will wear out underground hollows. When the water finds an outlet and drains away, it leaves a cave.

A karst is an area of limestone in which erosion has caused caves, sinkholes, natural bridges, fissures, and underground streams in the landscape. (Sinkholes occur when the roof of a cave wears too thin and collapses.) Note that canyons are made in a similar way as karst topography--by erosion--but have deep sides of exposed rock, carved out by rivers or streams.

You can demonstrate how erosion forms karsts, using a container of moist, packed sand and a squirt gun with water. What happens when you spray a steady stream of water at a point on the sand? Try to make tunnels, holes, and bridges using the water. Can you figure out a way to make a sinkhole?