Billy Doss, age 9, of Newport News, Virginia, for his question:

How long does it take a cavern to form?

The life span of a mountain lasts millions of years and a river may follow the same path for countless ages. Other geological features have shorter life spans. Lakes may fill and dry up in a few thousand years. Many caves also form and collapse in a few hundred years. On the slow, ancient clock of geology, this is just time enough to blink an eye. Other caves take longer, perhaps thousands of years to form. But at last they too collapse and disappear.

Caves tend to form in thick beds of soft limestones where seeping ground water can dis solve the rocky minerals. The rain trickles down through cracks in the ground and nibbles bigger and bigger holes below the surface. In a few hundred years, it can dig out a small cave. In perhaps 5,000 years, the seeping water may nibble out a huge underground city of connecting caves and caverns. Sometimes the pounding surf of the sea digs caves in the cliffs. These sea caves may become vast caverns and endure for more than 30,000 years.