Uncle points out how the rocks tilt. Earthquakes gradually push the crust's rocks on top of other rocks. So the deeper we go, the older the rocks become.
The really amazing thing is that these movements take millions of years. But as caves are formed, water brings soil into them, and the local plants and animals move in. Sometimes later movements shut the caves off from the surface. So could we find ancient animals living in ancient caves?
Some plants adapt to the dark. But uncle points out that the movement of rocks, combined with water vapour, could produce permanent clouds of lightning. He wonders if sufficiently large caves might have their own light?