Ronnie Low, age 9, of Collingwood, Ontario, Canada, for his question: '

What causes an oasis?

A desert is a dry, dry stretch of land, where hardly any rain falls at all. But this is not the whole story. Once in a while, there are desert showers and once in a while the rain dashes down in a deluge. Soon after it stops, all the water disappears or so it seems. Actually a lot of it sinks down below the surface. There it collects among the deep rocks and gravel below. This buried water is called ground  water. And there is plenty of it, even under the driest deserts.

In host deserts the ground water is buried very deep, often more than 300 feet down. Put in a few places, the top level of the groundwater comes almost to the surface. It forms an oasis where the roots of trees can grow down and drink. People dig wells and draw up this ground water. Often they pipe it around the oasis to water their fields and gardens.