HISTORY OF THE SAHARA:

TO MOST OF US, THE SAHARA DESERT CONJURES UP IMAGES OF A PRACTICALLY
UNLIVABLE EXPANSE OF LAND. ITS BIGGER THAN THE WHOLE UNITED STATES. ITS
SCORCHING HOT. ITS PARCHED DRY.   BUT IT WASN'T ALWAYS THAT WAY, SAYS
GEOLOGIST JOHN MCCAULEY.
     
MCCAULEY:  "Up until the recent past there have been many interludes in the history of the sahara where
the climate was semi-arid to relatively humd and during those periods of times it was a grassland and there
were millions of animals." 

TO HELP SEARCH FOR TRACES OF THIS WET WONDERLAND THAT THE SAHARA USED TO
BE, MCCAULEY'S GONE RIGHT TO THE TOP.  100 MILES ABOVE THE EARTH TO BE EXACT. 
HE RUNS EXPERIMENTS ON THE SPACE SHUTTLE TO SEARCH FOR ANCIENT RIVER BEDS. 
THE SHUTTLE SENDS A RADAR SIGNAL DOWN TO THE SAHARA, SOME OF THE RADAR
BOUNCES OFF THE SAND, BUT THE REST GOES STRAIGHT INTO THE GROUND, ONLY
BOUNCING BACK IF IT HITS SOMETHING HARD BURIED IN THE SAND, LIKE THE ROCK OF
A RIVER BED.  BY WATCHING HOW THE RADAR WAVES BOUNCE BACK, A COMPUTER ON
THE SHUTTLE CAN MAP OUT WHAT'S UNDER THE SAND. 

MCCAULEY: "these thin blankets of sand cover the geology immediately below the surface and the great
strength of the radar system is that it tis capable of seeing through this layer therby allowing us to
recognize . . . certain patterns. . . which are indicative of their having been ancinet rivers underneath the
sand." 

MCCAULEY INTERPRETS THE SPACE SHUTTLE PICTURES TO SHOW TRACES OF THREE
HUGE RIVERS--UP TO THREE MILES ACROSS--THAT ONCE FLOWED OVER NORTHERN
AFRICA.  NEXT HE'D LIKE TO USE THE ShUTTLE TO GET SOME IDEA OF HOW THE SAHARA
TURNED INTO THE DESERT WE SEE TODAY.