http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16963
How does a scientist know that the vast, featureless sand fields of the
Sahara once ran with water? Water leaves its mark on the land, cutting
channels and canyons where rivers flowed and leaving depressions where
lakes once pooled. In the Sahara, however, these features are filled with
sand. A scientist would have to look under the sand to find the imprint of
water from wetter climates. These images of the Safsaf Oasis in
south-central Egypt do just that: the top image shows the surface of the
desert, while the lower image reveals the water-carved rock under the
sand.