The Rio de Oro was the southern of two territories which once formed part of the Spanish colony of Spanish Sahara, which Spain ruled from 1884 to 1975. Today it is part of Western Sahara. It lies between 26° to the north and 21° 20' to the south and is about two thirds of the area of Western Sahara. The climate is arid and much of the landscape is as described by Johns, a desolate and arid desert, typical of the Sahara.
In The Case of the Visiting Sultan, Biggles determined that this was the most likely area where Rocky Cordova would attempt to ambush the aircraft of Sultan Oba I'Mobi as he flew from Masdu in Lashanti to London carrying his crown jewels. Biggles flew a decoy aircraft one hour ahead and, as expected, was ambushed over the Rio de Oro. In the ensuing dogfight, Biggles shot down Cordova's aircraft, killing the master criminal and his pilot Juan Laroula.