Larry Roberts, age 8, of Spokane, Washington, for his question:

What do they mean by an underground river?

We see rivers flowing along on top of the ground outdoors, under the wide open sky. It seems downright impossible for a river to flow under the ground, buried down out of sight. Nevertheless, it is possible. There really are underground rivers. We have found a lot of them and there are many, many more that nobody has discovered. These underground rivers are filled with rainwater that sinks down, down into the spongy rocks below the surface. Some of this water fills the pores and pockets of buried rocks. It collects in vast stores of buried ground water.

But whatever it is, water always must flow downhill. If it is on a slope, it must gush or trickle down to lower land. The buried ground water also must flow downward. Sometimes it collects in tilted rocks buried deep in a hill. Then it must flow down a slope. As it flows it chews and washes out a tunnel for itself through the rocks. This flowing, underground water is an underground river. Sometimes it digs its way to the side of a slope and comes gushing outdoors in a stream or a waterfall. Sometimes it flows on and on for miles underground.