Betty Jo Simas, age 10, of West Warwick, R.I., for her question:

IS THERE REALLY WATER UNDER THE EARTH?

The planet earth, as we know, is a solid round globe made of very heavy materials. We live on the surface layer of crusty rocks. This outer crust forms the continents and dips down to form the floors of the oceans. Its thickness varies from five to 40 miles or so. And everywhere below the earth's dry surface there are supplies of buried water.

When you walk on the sandy Sahara desert, you might never suspect that there is water down under your feet. But it is there, though maybe a mile or more under the surface of the earth. We call it groundwater because it is trapped and buried within the rocks of the earth's crust.

In a region riddled with caves, this buried water may form underground lakes and streams. But this is unusual. Most of it is trapped in the small pockets of porous rocks. These buried layers may be saturated like rocky sponges.

About half of the earth's groundwater is in the upper half mile of the crust. The rest is buried at various levels to depths of three miles. Groundwater near the surface often gushes forth in streams and bubbling springs. When the rocks below were heated by old volcanos, the springs erupted in steamy geysers.  Streams from cool springs slide down the slopes to join the rivers on their way to the sea. Geologists estimate that two million cubic miles of groundwater are down there at various levels in the earth's crust.  And each cubic mile of water is enough to fill a square tank that measures one mile on all sides. This enormous supply of groundwater is part of the never ending water cycle that sheds 95,000 cubic miles of moisture every year. Some of this rainfall runs over the surface in streams to join the ocean. Some is absorbed by plants from the soggy soil. The rest sinks down through the crust to percolate through porous rocks. In rainy seasons, the upper level of the groundwater rises, and in periods of drought it sinks deeper below the surface.