You might mean a mirage.
An illusion
A mirage is a desert illusion.
mirage, illusion
There are two other deserts in Africa besides the Sahara - the Kalahari and the Namib.
The likely word is mirage (optical illusion, notably in the desert).
Mojave Desert Great Basin Desert Chihuahuan Desert Colorado Plateau Desert
The Sahara is the largest desert outside of Antarctica.
The Sahara is the second largest desert on earth.
the desert is called the desert
Sometimes under the ground depending on the animal or desert.
I think you are looking for something called an "oasis".
Ayers Rock (properly called uluru) is the big rock in and of itself, its something in a desert, not the desert itself.