John W. Corbett, age 14, of Summerville, Penna., and to Sherry Williams, age 13, of Albertville, Ala., for their question;

What causes limestone caves?

Running water will, In time, wear away the hardest of rocks   and limestone is a softish rock made from chemicals which have a tendency to dissolve in water. Rainwater nibbles cracks in a bed of limestone more easily than in a layer of, say, granite. In time, the drip dripping ground water enlarges these cracks into underground holes as the limestone chemicals are dissolved and carried away.

The buried holes grow into caves and caverns. Underground streams bore out tunnels. Some of the ground water, loaded with chemicals, evaporates in the cool air of the chambers. The chemicals stay behind and rebuild themselves to form stalagmites, stalactites and other stony designs.