Becky Ruskoff, age 10, of Niagara Falls, Ontario, Canada, for her question:

Why are caves so wet and damp?

Caves are carved into earthy rock by moving waters. Some are created by seeping, drip dripping water, others by the pounding waves of the sea and a few are made by melting ice. All caves are formed by water and the water never stops work until at last they collapse. This is why caves and caverns are so damp and wet. The water that forms them also fills and chills the air inside with moisture.

The earth's caves are not meant to last as long as the mighty mountains. They are rooms and tunnels carved into the rocks below the surface. Sooner or later they get too big or their roofs become too thin. Then the force of gravity pulls them down and they collapse into hollow pits. No sensible person goes exploring caves without expert advisors and rescue equipment along on the trip. Who knows, an unexplored cave may be dust ready to come tumbling down in a pile of rubble.

But there are plenty of caves that have safe, guided tours for the public. They also have lights to show us the wonders inside. A large cave system is a strange underground world with enormous caverns, often on different levels. There may be underground streams flowing through tunnels and underground lakes and puddles filled with water.

There may be stalactites, hanging from the ceilings like giant icicles made of stone and spikey stalagmites poking up from the floor. There may be delicate columns from floor to ceiling and a lot of elegant stone carvings on the walls. All these stony surfaces are damp with moisture and here and there they gleam with films of dribbling water.

As a rule, these wondrous caves are carved into thick beds of limestone. This mineral gets riddled with pores and pockets because its chalky chemicals dissolve in water and get washed away. After every shower, raindrops seep down    down through the spongy rock, wash through the holes and make them bigger. After a few hundred years, the holes in the buried limestone grow to be enormous caves and caverns.

In time, they may form a vast underground city. But the soft limestone is still there and the drops of rain still seep down from the surface. This water drips through the ceilings of the caves and dribbles down the walls. It keeps all the stonework damp with dewy moisture. Some of it trickles into puddles and pools that evaporate and add more moisture to the air.

The chalky chemicals do not stay dissolved in the limestone forever. When the water trickles slowly or gets trapped in a puddle, its limey fragments are dumped. Gradually they build up layers of solid rock in icicles of stone and form artistic stone carvings on the walls.