Long, long ago, the Sahara was not quite so dry as it is now. There
were rivers then, which have dried up since. When there was water,
food would grow, and people could keep sheep and cattle. In those
days there were several large cities there. But when the water began
to dry up, the ground became sandy and nothing would grow. Then,
whenever the wind blew, the sand was carried along and began to cover
up the houses and temples. The people had moved away because their
food would not grow, and soon the sand completely covered the old
cities. For a long time they were buried, until some Europeans went
to see what they could find out about the people who lived there long
ago. Then they dug and dug in the sand, and found the old houses and
temples. But digging in the desert is very hard work, because it is
very hot, and there is very little water and food. Often, too, a
great wind arises and brings a sandstorm. Then the sand drifts back
again to the places already cleared.